A5 Study Master Brakes
Proportioning, metering, and pressure-differential valves
A combination valve could include__________________.
Answer B is correct. When performing a decelerometer test you are testing the stopping power of the brake system. Most manufacturers recommend accelerating the vehicle to 20 mph and applying the brakes in a firm application on a dry, level surface, free of gravel, debris and other obstacles. Skidding should be avoided as it can skew the results. The best coefficient of friction is obtained just before the point of wheel lock up or skidding. This takes into account why ABS are now mandated on heavy duty transit vehicles.
A decelerometer like the one in the figure is being used to check the performance of a pre-ABS equipped air brake system. Technician A says the bus should be driven to 20 mph in a safe area and the brakes applied until the wheels lock up or skid. Technician B says the bus should be driven to 20 mph in a safe area and the brakes should be applied quickly and firmly but not to the point of wheel lock up. Who is correct? A. A only B. B only C. Both A and B D. Neither A nor B
Answer C is correct. When a hold-off diaphragm leaks, the result can be insufficient air pressure to prevent the power spring from loading the chamber pushrod applying brake pressure. So Technician A is right. Technician B is also right; a broken or weak return spring can cause brake drag.
A driver complains ofdragging brakes in an air brake equipped bus. Technician A says a leaking hold-off diaphragm could cause the problem. Technician B says a broken return spring in the service chamber could be the cause. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Has less traction
A locked wheel ___________________ to the road surface than a rolling wheel.
Maintain a slight pressure on the hydraulic system
A residual check valve is used to ______________.
Answer D is correct. When performing an air pressure balance test the test gauges should be located at two corresponding brake chambers because the pressure is important but so is the timing of the delivery. The distance the air has to travel and the condition of the lines and fittings will affect this, so the gauges should be as close to the brake assemblies as possible.
A technician is performing an air pressure balance test between the front axle and rear axle of a bus. Where should the pressure gauges be located? A. At the brake application valve B. At two corresponding relay valves C. At each of the corresponding reservoirs D. At two corresponding brake chambers
Answer C is correct. Both Technicians are correct. Technician A is correct because the purpose of the air drier is to remove the moisture and some passed oil. A defective unit could definitely cause this problem. Technician B is correct because most air driers use the governors unloader circuit signal (or air pressure) to signal the air drier when to purge and expel the contaminants.
A transit vehicle equipped with an automatic air drier is found to have excessive moisture and oil contamination when the air tanks are drained on a preventive maintenance inspection. Technician A says this could be caused by a defective air drier unit. Technician B says the air governor could be defective. Who is correct? A. A only B. B only C. Both A and B D. Neither A nor B
Normally open
A typical brake light is electrically _________________.
Low spongy brake pedal
Air in the lines will cause what type of problem?
Answer B is correct. A PBBT measures brake forces at each wheel. Electronic testers, such as Vericom and Fraser, sense rate of speed change and then uses this data to calculate stopping distances and a mechanical tester such as Tapley meter is a pendulum device that measures deceleration but only saves a value that represents the maximum deceleration reached during the stop.
All of the following are common brake testers EXCEPT: A. performance-based brake testers. B. hydraulic tester. C. electronic tester D. mechanical tester
Answer A is correct. There is no cam actuator used on an air disc brake system.
All of the following are parts of an air disc brake assembly EXCEPT: A. cam actuator. B. torque plates. C. calipers. D. rotors.
Answer D is correct. All of the answers relate to brake torque imbalance problems except for a vehicle overload condition. While overload might tax brake system potential, it would not in itself cause brake torque imbalance.
All of the following can be causes of brake torque imbalance EXCEPT: A. oil-soaked linings. B. nonuniform adjustment. C. nonuniform chamber sizes. D. an overloaded vehicle.
Answer D is correct. When a dual-circuit, foot application valve is changed, the air system should be drained entirely, so maintaining brake system pressure is not a requirement of the procedure.
All of the following steps in removing a dual-circuit, foot application valve are correct EXCEPT: A. set the parking brake. B. label the brake lines. C. mark the valve body in relation to the mounting plate. D. maintain brake system pressure.
Answer D is correct. This type of valve opens automatically from the changing pressure in the reservoir, allowing water and contaminants to escape through the valve to the atmosphere. If the pressure in the valve S (supply) port is higher than the pressure in the valve sump, air moves past the valve into the sump. Water and contaminants are carried with the air into the valve sump. If the pressure in the sump is 2 psi (13.79 kPa) higher than the pressure at the S port, this pressure forces the valve upward and opens the exhaust port. When this valve opens, water and contaminants are expelled to the atmosphere. As soon as the pressure in the sump drops below the pressure at the S port the valve closes.
An automatic reservoir dmin valve as shown in the figure does which of the following? A. It opens when the reservoir pressure reaches the governor pressure. B. ft is mounted in a threaded fitting near the top of the reservoir. C. It opens by decreasing supply reservoir pressure. D. Drains water ifthe valve's sump pressure is 2 psi higher than reservoir pressure
Technician A only
An owner of a vehicle equipped with ABS brakes complained that whenever he tried to stop on icy or slippery roads, the brake pedal would pulse up and down rapidly. Technician A says that this is normal for many ABS units. Technician B says that the ABS unit is malfunctioning. Which technician is correct?
The boiling temperature decreases.
As brake fluid ages, what happens?
When driving through water puddles or during a severe rainstorm
Brake fade caused by water can occur __________________
PSI
Brake fluid pressure is measured in what unit?
1,300ºF (700ºC)
Disc brake pads can reach temperatures as high as _____________.
Pads
Disc brakes use replaceable friction material called __________.
Shoes
Drum brakes use replaceable friction material called ________.
Answer A is correct. A restricted air inlet will increase air pressure buildup time. Answer B is wrong. The spring brake chamber is only charged with air when the parking brakes are released. Answer C is wrong. The secondary service line is only charged with air when the service brakes are applied. Answer D is wrong. A restricted governor line will only affect governor operation.
During a brake inspection, a technician tests the air supply system and finds that buildup is slow. Which of these is the Most-Likely cause? A. Clogged compressor inlet filter B. Leak in a spring brake chamber C. Air leak in the secondary service line D. Restriction in the governor line
Answer B is correct. Oil saturated or glazed lining should be replaced, not cleaned, so Technician A is wrong. Technician B is right; it is good practice to replace the linings on the brake along with those on the other side of the axle to ensure good brake balance.
During a wheel seal repair, the brake friction facings are observed to be saturated with oil and glazed to a mirror finish. Technician A says that the linings should be cleaned with liquid detergent and power washing. Technician B says that the brake linings should be replaced along with those on the other side ofthe axle. Who is right? A. Aonly B. B only C. Both A and B D. Neither A nor B
Both the accelerator and the brake pedal
Electric adjustable pedals operate___.
Worn outer ribs
Flexible brake hose should NOT be inspected for :
Answer D is correct. Because the governor cutout is set correctly and cut-in is out of specification, the governor has to be changed to prevent frequent cycling of the compressor.
Governor cutout is set at 125 psi on a bus air system. However, governor cut-in occurs at 115 psi. Which of the following would be Most-Likely to repair the problem? A. Adjust governor cut-in to 105 psi. B. Check the operation of the pop-off valve. C. Replace the compressor. D. Replace the governor.
Enough to allow all brakes to become completely worn
How much brake fluid is in a typical master cylinder reservoir?
Answer D is correct. The figure shows a standard dash park control valve, which controls air supplied to the spring brake hold-off circuit. Pulling it out exhausts air from the hold-off circuit placing the vehicle in park mode.
In the figure, when the park control valve is pulled out: A. the parking brake releases. B. air builds up in the piggyback. C. it will not reopen until the treadle valve is depressed. D. air in the spring brake chambers is exhausted.
Answer D is correct. Technician A is wrong because draining one side will not drain all the gear oil from the differential and both planetary gear hubs; they must be independently drained and filled. Technician B is wrong because you must fill and check the gear oil level separately.
In the pictures above, Technician A says draining fluid from one side will drain all the gear oil from both differential and both planetary gear hub ends. Technician B says you must check and fill gear oil from just the differential; this will fill both planetary hub ends. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
16 to 32 ft/sec2
Maximum deceleration rates for a typical passenger car or light truck is __________.
Answer B is correct. It represents the color coding used mostly by vehicle manufacturers as applied to the vehicle and on pneumatic schematics. The other choices are incorrect.
Most bus manufacturers will color code the nylon air tubing used within the vehicle. The color coding represents: A. red= primary brake, green= secondary brakes, brown= emergency / parking brake circuit. B. red= front brakes, green= rear brakes, brown= parking brakes. C. brown= emergency, red= secondary brakes, brown = parking brakes. D. red= supply circuit, brown= primary brakes, green= secondary brakes.
Answer C is correct. Expensive engine compressor drive components can be protected by using a sacrificial fiber or plastic type drive gear. In the event of a compressor lockup the drive gear offers the weak link verse the engine drive gears. All other answers are incorrect.
Nonmetallic-style drive gears are sometimes used to couple an air compressor to the engine gear train because this: A. allows for alignment irregularities. B. absorbs compressor torsional vibration. C. will protect the engine gear train in the event of a catastrophic compressor failure. D. is used as a weight-saving measure.
Answer D is correct. Of the conditions listed, the excessive padwear could only be caused by a caliper assembly that does not slide freely.
On the air disc brake as shown in the figure, what would Most-Likely cause excessive wear on the brake pad identified? A. Piston seized in its bore B. Damaged rotor C. Defective brake hose D. Seized caliper slide pins
Neither Technician A nor B
Technician A says a pull to the right during braking could be caused by a defective metering valve. Technician B says a pull to the left could be caused by a defective proportioning valve. Which technician is correct?
Answer D is correct. Neither Technician is correct.
Technician A says both front and rear wheel bearings are adjustable. Technician B says you use a brass drift to remove a rear wheel bearing race. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Technician B only
Technician A says that a power-assisted brake reduces stopping distances compared with a non-power-assisted brake system. Technician B says that the power-assisted brake system reduces the force that the driver must exert on the brake pedal. Which technician is correct?
Answer B is correct. Technician A is wrong. The only adjustment on an air pressure governor is the cutout pressure, so if the cutout pressure is set correctly and the cut-in pressure is out of specification, the governor should be replaced. Technician B is correct. The difference between governor cut-in and cutout should be between 20 and 25 psi.
Technician A says that air governor cut-in pressure should be adjusted before the cutout pressure. Technician B says that the difference between governor cutout and governor cut-in should be between 20 and 25 psi. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Technician B only
Technician A says that all vehicles equipped with ABS will experience a pulsating brake pedal even during normal braking. Technician B says that some ABS units only control the rear wheel brakes. Which technician is correct?
Answer A is correct. Technician A has made a correct statement about retarder and ABS operation. Technician B has failed to describe correct retarder and ABS operation.
Technician A says that an ABS occurrence will automatically disable the retarder. Technician B says that the retarder and the ABS system work in conjunction to slow the vehicle. Which Technician is correct? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. Technician A is correct. If a low-pressure warning light does not illuminate when system pressure drops below 60 psi, the problem could be as simple as a defective bulb. Technician B is wrong in saying a defective dash pressure gauge could cause this problem because the low-pressure warning light circuit must be separate from the dash pressure gauge circuit.
Technician A says that an inoperative warning lamp below 60 psi could be caused by a defective warning light bulb. Technician B says that an inaccurate dash pressure gauge can cause a warning light to fail. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer C is correct. Technician A is correct. Daily draining of air reservoirs not equipped with spitter valves is a recommended practice. Technician B is also right; automatic drain valves should be periodically checked. This periodic check should involve completely draining the tanle
Technician A says that daily draining of air reservoirs not equipped with automatic drain valves is recommended. Technician B says that automatic drain valves and moisture-removing devices should be checked periodically for proper operation. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Technician A only
Technician A says that glycol-based brake fluid causes the seals to swell slightly. Technician B says the DOT 5 (silicone-based) brake fluid causes rubber seals to swell slightly. Which technician is correct?
Both Technicians A and B
Technician A says that some proportioning valves are one part and used to control both rear brakes. Technician B says that some proportioning valves are two parts, one for each rear wheel brake. Which technician is correct?
Technician B only
Technician A says that the front brakes do most of the braking because the front brakes are larger. Technician B says that due to weight transfer, most of the braking force needs to be supplied by the front brakes. Which technician is correct?
Both Technicians A and B
Technician A says that the red brake warning light can be turned on if a difference in pressure is detected by the pressure-differential switch. Technician B says that the red brake warning light can be turned on if the brake fluid level sensor detected low brake fluid level. Which technician is correct?
Both Technicians A and B`
Technician A says that the thicker or heavier the disc brake rotor, the more heat can be absorbed. Technician B says that the faster the vehicle is traveling when the brakes are applied, the greater the amount of heat created in the brake system. Which technician is correct?`
Answer A is correct. Slack adjusters are levers; make sure that the specified clevis pin-to-camshaft dimension is always observed. A small difference in slack adjuster effective length can make a big difference in brake torque.
Technician A says that when replacing a slack adjuster, the new one must have the same dimension between the center of the S-camshaft to the center of the clevis pin. Technician B says that when replacing a slack adjuster, increasing the length is acceptable because it increases brake torque at the wheel. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
The Performance of the Braking system
The FMVSS 135 standards determine ____________.
Increase the driver's force on the brake pedal applied to the master cylinder.`
The brake pedal assembly uses a mechanical lever to __________________
Both Technicians A and B
The brakes on a vehicle work okay for a while, then the vehicle slows because the brakes self-applied. Technician A says that an overfilled master cylinder could be the cause. Technician B says that a blocked vent port (compensating port) could be the cause. Which technician is correct?
Metering
The button on the _______________ valve should be held when pressure bleeding the brakes.
Answer C is correct. Because the vehicle is moving, it indicates the parking brakes are released meaning that there is air in the spring brake chambers; when the driver makes a service application, air is now charged to the service chambers. Air is present in both the service and spring brake chambers.
The driver of an air brake-equipped bus traveling down a highway makes a full service application of the brakes. At the time of the service application, which brake chambers on the vehicle are charged with air? A. Service chambers only B. Spring brake chambers only C. Service and spring brake chambers D. All chambers are evacuated
Answer C is correct. Technician A is describing correct procedure to be observed when installing a wheel assembly to an axle. Technician B is also correct; most bus drive axle wheel ends are lubricated by oil from the differential carrier through the axle housing.
The dual wheel assembly of a bus axle is being reinstalled. Technician A says to put a light coating of lubricant on the spindle and place a seal protector over the threads. Technician B says rear drive wheel hub assemblies are lubricated by oil from the differential carrier. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Speed difference between the two surfaces.`
The friction between two surfaces is NOT affected by ______________.
Answer B is correct. When the low-air pressure warning fails to extinguish after startup it would mean that system pressure was not higher than 60 psi in either or both of the onboard circuits; after startup, the operator would have to attempt to build system pressure before making a judgment on whether there was a malfunction, so Technician A is wrong. Technician B is correct in saying that a short circuit could cause the low-pressure warning light to stay on even if system pressure was above 60 psi.
The low-pressure warning light on a vehicle dash does not go out after startup. Technician A says that the problem may be caused by a loss ofone section of the dual system. Technician B says that the problem may be caused by a short circuit. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
One teaspoon
The master cylinder is able to move about how much brake fluid during each brake application?
Answer D is correct. When the parking brake fails to hold during yard testing, the first thing to check is the slack adjuster free stroke; remember that parking brake effectiveness depends entirely on brake adjustment.
The parking brakes will not hold when yard tested on a bus. Which of the following should be done FIRST? A. Pull the wheels and replace the brake linings. B. Replace the bus spring brake control valve. C. Test the bus air supply pressure. D. Check bus slack adjuster free stroke dimensions.
Technician A only
The primary brake circuit fails due to a leak in the lines, leaving the rear section of a dual split master cylinder. Technician A says that the driver will notice a lower than normal brake pedal and some reduced braking power. Technician B says that the brake pedal will "grab" higher than normal. Which technician is correct?
Technician B only
The rear brakes lock up during a regular brake application. Technician A says the metering valve could be the cause. Technician B says that stuck front disc brake calipers could be the cause. Which technician is correct?
Technician B only
The rear wheels lock up during hard braking. Technician A says that a defective metering valve could be the cause. Technician B says that a defective proportioning valve could be the cause. Which technician is correct?
Answer D is correct. The consequence of a broken return spring in a foundation brake assembly is the brakes will drag because the function of this spring is to pull the shoes away from the drum after a brake application.
The return spring breaks in the foundation assembly of a bus S-cam system. What is the likely consequence? A. Delayed application B. Reduced brake torque C. Parking brake lag D. Dragging brakes
Double flare and ISO
The two types of flares used on brake lines are _______________.
DOT3
The type of brake fluid most often recommended by vehicle manufacturers is _______.
True
True or False: Kinetic energy of a vehicle doubles when the speed doubles.
Both Technicians A and B
Two technicians are discussing a problem where the brake pedal travels too far before the vehicle starts to slow. Technician A says that the brakes may be out of adjustment. Technician B says that one circuit from the master cylinder may be leaking or defective. Which technician is correct?
Both Technicians A and B
Two technicians are discussing master cylinders. Technician A says that it is normal to see fluid movement in the reservoir when the brake pedal is depressed. Technician B says a defective master cylinder can cause the brake pedal to slowly sink to the floor when depressed. Which technician is correct?
Technician B only
Two technicians are discussing why the brake lines are coiled near the master cylinder on many vehicles. Technician A says that these coils help prevent moisture from flowing from the master cylinder to the calipers. Technician B says that they help prevent cracks that could be caused by vibrations. Which technician is correct?
Answer D is correct. With the scan tool and PC, a technician cannot view the sensor output voltages. Most scan tools will only read DC voltages. With PCs with the manufacturer's software, a technician will be able to view a sensor's revolutions per minute (rpm) and the vehicle's miles per hour (mph) only. With a scan tool and PC, a technician is able to check the engine data links and relays, vehicle voltages, the ABS program identification, the system's configuration, perform component tests, and view active and inactive faults.
Using the ABS/ATC scan tool or PC, all of the following can be checked EXCEPT: A. engine data link and relays. B. vehicle voltages. _ C. view program ID information. D. sensor output voltage.
Select a lower transmission gear
What can the driver do to reduce the possibility of brake fade caused by heat?
Answer A is correct. An inversion valve is a failure back-up device that helps control braking in the event of a catastrophic primary brake system failure by modulating the hold-off pressure in the spring brake chambers; this means that actual braking force is provided by the chamber power spring.
What force in spring brake chambers is being used to brake the vehicle when an inversion valve is operating during a failure? A. Spring brake power spring B. Service air C. Service chamber retraction spring D. Hold-off air
Brake Noise Levels
What is NOT specified by the FMVSS 135?
Answer D is correct. The LEAST-Likely cause of water in an air reservoir is continuous operation of the system in humid conditions, because the system when functioning properly is designed to handle conditions of high humidity. The other statements are possible causes.
What is the LEAST-Likely cause of water in air reservoirs? A. Air drier not purging B. Lack of periodic drainage of supply tanks C. Oil-saturated air drier dessicant D. Continuous operation in humid conditions
Answer C is correct. Faulty valves will usually cause airflow problems and air leaks at the valve. The brake chambers are sealed units. When a leak occurs at the cage bolt hole or key hole, this is caused by a leaking or tom hold-off diaphragm. A damaged service diaphragm would create a leak at the pushrod hole.
What is the Most-Likely cause of air leaking from the cage bolt hole (key hole) of a spring brake chamber as shown in the figure? A. A faulty quick release valve B. A defective application valve C. A punctured hold-off diaphragm D. A punctured service diaphragm
Answer C is correct. The component shown in the figure is the service retraction spring.
What is the part marked "A" in the figure shown? A. Hold-off diaphragm B. Power spring C. Service retraction spring D. Pressure plate
To restore like-new performance
What is the purpose of any brake repair?
Rubber diaphragm or floating disc
What part in the master cylinder helps keep air from contacting the brake fluid yet allows the fluid level to drop as the disc brakes wear?
Answer A is correct. All the answers relate to the practice of removing a piggyback assembly on a spring brake except for removing the spring brake chamber clamps. Current practice is to NEVER separate the spring brake (parking brake) chamber for any reason.
What should you NOT do, when replacing the diaphragm on a service brake chamber? A. Remove the spring brake chamber clamps B. Cage the spring brake C. Loosen the service chamber clamps by tapping them with a mallet D. Exhaust the air pressure
Rotor Fade
What type is not a type of Brake Fade?
Answer B is correct. More of gap between the sensor and the reluctor wheel, the lower the output voltage. Answers C and D are wrong. The sensor produces an AC output voltage. Answer A is wrong. An increase in voltage would be a result of a sensor closer to the tone/reluctor wheel.
What would be a result of a technician installing a wheel speed sensor more than specified air gap? A. Output voltage would increase. B. Output voltage would decrease. C. Input voltage would increase. D. Input voltage would decrease.
Answer C is correct. Technicians A and B are accurately describing steps in the TMC wheel end procedure, which every truck technician should know.
When adjusting wheel bearings, Technician A says torque the adjusting nut to 200 ft.llbs, then back off one turn, re-torque to 50 ft.llbs, and perform a specified final back off followed by an end play check. Technician B says wheel end play should be measured with a dial indicator. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer C is correct. This question is asking you to identify the incorrect statement and the cam profile should never be lubricated with anything.
When assembling an S-camshaft, all of the following apply EXCEPT: A. both camshaft seal lips must face toward the slack adjuster. B. use the proper driver when installing camshaft seals. C. lubricate the cam profile with petroleum jelly. D. measure camshaft radial movement.
Answer B is correct. The maximum allowable S-cam radial free play is 0.030 inch (0.76 mm). Any higher readings require S-cam bushing replacement.
When checking S-cam bushings for radial free playas shown in the figure, what is the maximum reading allowed before the S-cam bushings must be replaced? A. 0.Gl5 inch B. 0.030 inch C. 0.450 inch D. 0.060 inch
Answer A is correct. Test specification for air pressure variation from axle to axle on the same vehicle is no more than 4 psi.
When checking air pressure brake balance on a bus, what is the maximum permitted variation from axle to axle? A. 4 psi (27.6 kPa) B. 8 psi (55.2 kPa) C. 12 psi (82.7 kPa) D. 16psi(1l0.3kPa)
Answer A is correct. The simplest, easiest, and least expensive solution for checking air tanks for leaks is soap and water. The soapy solution will form bubbles at the point of the air leak.
When checking air tanks like the one shown, what should be used to check the tank for air leaks? A. A soap and water solution B. Methyl hydrate C. Die penetrate D. An ultra sonic tester
Answer D is correct. Technician A and B are both wrong. If an automatic slack adjuster stroke is found to be over the legal limit, it is first recommended to check the foundation brake components and the proper installation of the slack adjuster. Worn brake components such as S-cam and anchor pin bushings could cause the adjuster to have an excess stroke. A loose control arm fastener would also increase the stroke. Only after verifying that the foundation brake components and slack set-up are correct is when a technician should check for a defective automatic slack adjuster.
When checking the stroke on an automatic slack adjuster, Technician A says replace the slack adjuster if the stroke is found to be over the legal limit. Technician B says if the slack adjuster is found to be over the legal limit, adjust the slack adjuster to specification. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer B is correct. A defective bearing adjustment can contact the sensor and increase the sensor/tone wheel gap. Technician A is incorrect because the first step in diagnosing a sensor fault would be to check the specified sensor resistance with an ohmmeter. The next would be to check the sensor AC voltage output by rotating the wheel. A reading of.2 volts AC at 30 rpm would verify a good output signal.
When diagnosing a sensor fault. Technician A says, first check the wiring harness and verify good continuity. Technician B says a defective wheel bearing adjustment on the drive axle could put on the ABS fault light. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer C is correct. Both technicians are correct. Slack adjusters that are out of adjustment demand greater brake chamber travel, which requires a larger volume of air. A build-up of moisture in the reservoirs decreases the volume of reserve air so each brake application will use up more of the volume available.
When discussing excessive air pressure drops during routine stops, Technician A says that the slack adjusters may be out of adjustment. Technician B says that it could be caused by a build-up of moisture in the reservoirs. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. Technician B is wrong. The quick-release valve is utilized so that an additional avenue for compressed air to escape is present when the brakes are released.
When discussing quick-release valve diagnosis, Technician A says that when the service brakes are applied, the quick-release valve exhaust port should be closed. Technician B says that when the service brakes are released, the air pressure in the front brake chambers should be exhausted only at the treadle valve. Who is fight? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. The number stamped on the brake drum is the brake drum maximum wear diameter, Technician B is wrong because it is not necessary to replace brake drums at every brake reline; however, recent practice and the low cost of drums usually results in drum replacement with a brake job.
When discussing the condition ofa brake drum, Technician A says the number stamped on the drum is the brake drum maximum wear diameter. Technician B says the brake drum should be replaced when installing new brake shoes. Who is right? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. If the governor requires adjusting, remove the top cover from the governor and loosen the adjusting screw lock nut. Tum the adjusting screw counterclockwise to increase the cutout pressure. If this adjusting screw is rotated clockwise, the cutout pressure is decreased. Rotating the adjusting screw 1/4 tum changes the cutout pressure 4 psi (27 .58 kPa). Repeat the governor cutout pressure test to be sure the governor is adjusted properly. Recheck the governor cut-in pressure to be sure this pressure is correct. The governor cut-in pressure stays at a preset difference to cutout pressure.
When testing and adjusting the governor cutout and cut-in pressures, which of the following statements is true? A. Cutout pressure is increased by rotating the adjusting screw counterclockwise. B. Cut-in pressure must be adjusted before cutout pressure. C. Tum the adjustment screw 1/4 tum to change cut-in or cutout pressure 7 psi (48.27 kPa). D. Restricting the air line from the supply reservoir to the governor decreases cutout pressure.
Answer A is correct. Foot valve exhaust leakage should be tested when the valve is producing an application pressure of 80 psi or higher so Technician A is right. Technician B is wrong; testing of a toot valve is performed when malfunction is suspected or at a major PM inspection.
When testing foot valve leakage, Technician A says that the brake application pressure should be 80 psi (553.6 kPa). Technician B says that this test should be done every 50,000 miles. Who is correct? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. A quick-release valve is used to direct brake application pressure to the steering axle wheels on a bus so it is located close to the front service chambers.
Where is a quick-release valve mounted? A. Close to the front service chambers B. Close to the rear axle relay valve C. In the middle of the bus chassis D. On the outside back wall ofthe engine compartment
Answer D is correct. Brake compounding occurs when mechanical force applied by the power spring in the spring brake chambers and pneumatic force applied by the service chamber combine or "compound." This can result in an actuation pressure that exceeds the specified maximum by 150 percent, which can damage foundation brake components. Another way of saying this is the simultaneous application of spring and service brake forces acting on the foundation brakes.
Which of the following is known as brake compounding? A. Reversing primary and secondary brake lines on the treadle valve B. Running the stall test to evaluate parking brake condition C. Releasing the air in the spring brake chamber to allow the brakes to be applied D. Simultaneous application ofservice and spring brake forces to the foundation brakes
Lasts the life of the vehicle unless there is a leak Technician A says that DOT 5 brake fluid can be added to DOT 3 brake fluid. Technician B says that it is wise to purchase brake fluid that is in metal rather than plastic containers. Which technician is correct? Technician B only.
Which of the following is not a characteristic of conventional brake fluid?
Answer C is correct. Current practice is to replace either the spring brake assembly or install a piggyback unit if the hold-off diaphragm fails, mainly because it can be dangerous separating a spring brake at the hold-off chamber.
Which of the following is regarded as bad practice when serving a typical 30130 spring brake assembly as shown in the figure? A. Caging the power spring B. Replacing the service diaphragm C. Replacing the hold-off diaphragm D. Loosening the service chamber clamps to align the air ports
Answer A is correct. Overloading is the LEAST-Likely cause of a wheel bearing failure because bearings are specified for the expected load capabilities of a vehicle with a generous margin of error. Contamination, damaged race, and improper lubricant are more common causes of bearing failure.
Which of these is the LEAST-Likely cause of wheel bearing failure? A. Overloading B. Contamination C. A damaged race D. Improper lubricant
Front-wheel drive
Which type of vehicles most often do not use metering valves?
Answer B is correct. Technician A is wrong. Engine speed has nothing to do with the operation of the valve. Compressors that are fed already compressed air via the turbo charger, as high as 25 to 30 psi, utilize this valve to protect that boost pressure.
While discussing air drier operation, Technician A says that the turbo protection valve found on some air driers operates only at idle speeds. Technician B says that the turbo protection valve is used to protect turbo boost pressure while the air drier is purging. Which technician is correct? A. A only B. B only C. Both A and B D. Neither A nor B
Answer A is correct. The pressure-protection valve is a nornally closed valve designed to open at a pre-set pressure rating. Technician B is wrong because he is describing a pressure regulator type of valve.
While discussing the operation ofthe pressure-protection valve, Technician A says the valve is normally closed and opens at a pre-set pressure rating. Technician B says the valve is a normally open valve and acts as a pressure-reducing valve. Which technician is correct? A. A only B. B only C. Both A and B D. Neither A nor B
To help prevent damage to the brake hose
Why should brake calipers be supported by a wire during brake service and not allowed to hang by the flexible brake hose?
