Air Brakes-Section 5
Air Brakes During Step 5 Walk Around Inspection (1-7 of pre trip inspection) Check Slack Adjusters on S-cam Brakes by doing the following.
1) Park on level ground and chock the wheels to prevent the vehicle from moving. 2) Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can reach. 3) If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.
To Test Low Pressure Warning Signal
1) Shut the engine off when you have enough air pressure so that the low pressure warning signal is not on. 2.) Turn the electrical power on and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank (or tank with the lowest air pressure, in dual air systems).
To Check Air Compressor Governor Cut-in and Cut-out Pressures...
1) the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut-out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising 2) With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut-in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise. If it doesn't work the governor needs to be fixed to be safe to drive.
To Test Air Leakage Rate.....
1.) With a fully-charged air system (typically 125 psi), turn off the engine, release the parking brake, and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles. 2)Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than three psi in one minute for single vehicles (more than four psi for combination vehicles), the air loss rate is too much. Check for air leaks and fix before driving vehicle. Otherwise you could lose your brakes.
What is a low pressure warning?
A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi. (Or one half the compressor governor cutout pressure on older vehicles.)
What are spring brakes?
Are used for emergency and parking brakes and must be held on by mechanical force. When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes
Check Brake Drums (or Discs), Linings, and Hoses-Look for the following.
Brake drums (or discs) must not have cracks longer than one half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing Check the air hoses connected to the brake chambers to make sure they aren't cut or worn due to rubbing
To Check That Spring Brakes Come On Automatically.....
Continue to fan off the air pressure by stepping on and off the brake pedal to reduce tank pressure. The tractor protection valve and parking brake valve should close (pop out) on a tractor-trailer combination vehicle and the parking brake valve should close (pop out) on other combination and single vehicle types when the air pressure falls to the manufacturer's specification (20 - 45 psi). This will cause the spring brakes to come on.
What are the two causes brake fading or failure?
Excessive use of the service brakes results in overheating and leads to brake fade. Brakes out of adjustment
What do air storage tanks do?
Hold compressed air, enough air to allow the brakes to be used several times, even if the compressor stops working.
What is a front brake limiting valve?
In older trucks made before 1975 the valve control was in the cab. It had two settings: slippery and normal. It was intended to keep wheels from skidding on slippery surfaces but it's no necessary alway drive in normal.
To determine if your vehicle as ABS Breaks....
Look under the vehicle for the electronic control unit (ECU) and wheel speed sensor wires coming from the back of the brakes.
The low pressure warning signal is either a....
Red light and maybe a buzzer or a wig wag.
To test the Parking Brake
Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold.
Increasing application pressure to hold the same speed means?
The brakes are fading, slow down and use a low gear.
What is a stop light switch?
The switch turns on the brake lights when you put on the air brakes.
What to supply pressure gauges do
These gauges tell you how much pressure is in the air tanks.
What is an application pressure gauge?
This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.)
Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance =
Total Stopping Distance
A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc.
True
A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on.
True
Air Brakes During Step 2 Engine Compartment Checks (1-7 of pre trip inspection) check the air compressor drive belt if equipped with it, should be tight and and in good condition.
True
Air tank drains should be drained at the end of each day to remove oil and water.
True
All vehicles built since 1991 have automatic slack adjusters. Even though automatic slack adjusters adjust themselves during full brake applications, they must be checked.
True
Automatic adjusters should not have to be manually adjusted except when performing maintenance on the brakes and during installation of the slack adjusters.
True
Automatic air drain system have heaters so the don't freeze.
True
Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems
True
Check the alcohol container and fill up as necessary, every day during cold weather
True
Dual Parking Control Valves are spring loaded in the out position, pushing the knob in releases air from the separate air tank and releases the spring brake so you can move
True
Dual Parking Control Valves. When main air pressure is lost, the spring brakes come on. Some vehicles, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency.
True
If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring an emergency stop. Don't drive until you get the problem fixed.
True
If the air compressor has its own oil supply, the oil should be checked before driving.
True
In a vehicle equipped with automatic adjusters, when the pushrod stroke exceeds the legal brake adjustment limit, it is an indication that a mechanical problem exists in the adjuster itself, a problem with the related foundation brake components, or that the adjuster was improperly installed.
True
It is important to plan carefully when pushing the Dual parking control valve as this can only be done a few time.
True
Many vehicles have automatic front wheel limiting valves. They are not controlled by the driver.
True
Modulating Control Valves have a control handles on the dash and may be used to apply the brakes gradually if there is a service brake failure.
True
Older truck may have a lever for the parking brake.
True
Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes won't work.
True
Pushing the brake pedal down when the spring brakes are on could damage the brakes.
True
The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
True
The manual adjustment of an automatic adjuster should only be used as a temporary measure to correct the adjustment in an emergency situation as it is likely the brake will soon be back out of adjustment since this procedure usually does not fix the underlying adjustment problem.
True
The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem and is not fixing it.
True
The warning light and buzzer should come on before the air pressure drops below 60 psi
True
There are manual and automatic drain valves
True
Vehicles equipped with air brakes used a yellow diamond-shaped push pull control knob.
True
Vehicles with air brakes have more to inspect
True
Watch PSI gauges and pay attention for PSI warning light and buzzer. The warning light and buzzer should shut off at 60 PSI.
True
When you push the brake pedal down, two forces push back against your foot. One force comes from a spring. The second force comes from the air pressure going to the brakes. This lets you feel how much air pressure is being applied to the brakes.
True
When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.
Ture
To Test Service Brakes
Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about five mph), and apply the brakes firmly using the brake pedal.ack
To Check Rate of Air Pressure Buildup....
When the engine is at operating rpms, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (check specs on vehicle as this could vary due to larger tanks) In single air systems (pre-1975), typical requirements are pressure buildup from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 rpms.
How S-Cam Brakes work
When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting thebrake camshaft. This turns the s-cam (so called because it is shaped like the letter "S"). The s-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the s-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.
How disc breaks work
air pressure acts on a brake chamber and slack adjuster, like s-cam brakes. But instead of the s-cam, a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large c-clamp
Spring brakes will come fully on when...
air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi). Do not wait for the brakes to come on automatically. Stop right away
What does the service brake system do?
applies and releases the brakes when you use the brake pedal during normal driving.
What does the parking break do?
applies and releases the parking brakes when you use the parking brake control.
The need for increased pressure can be also be caused by:
brakes out of adjustment, air leaks, or mechanical problems
What does the air compressor governor do?
controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (around 125 pounds per-square-inch or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.
If only on part of the system is low on PSI it's ok to keep driving
false
What does an alcohol evaporator do?
helps to reduce the risk of ice in air brake valves and other parts during cold weather.
What does the safety valve do?
installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. Usually set to 150 PSI, if it open, something is wrong, have it fixed.
What does the air compressor do?
pumps air into the air storage tanks (reservoirs)
How Wedge Brakes work
the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum.Wedge brakes may have a single brake chamber, or two brake chambers, pushing wedges in at both ends of the brakeshoes. Wedge type brakes may be self- adjusting or may require manual adjustment.
The secondary system on dual air brake systems typically operate?
the regular brakes on the front axle (and possibly one rear axle)
The primary system on dual air brake systems typically operate?
the regular brakes on the rear axle or axles.
The air brake lag distance at 55 mph on dry pavement adds about 32 feet
true
What does the emergency break do?
uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
If the low pressure signal does not work, you could
you could lose air pressure and you would not know it. This could cause sudden emergency braking in a single-circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.
