CDL Air Brakes (Section 5)

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What makes air brakes work?

Compressed air.

What are the three braking systems?

Service brake, parking brake, emergency brake

What does the service brake do?

Applies and releases the brakes when you use the brake pedal during normal driving.

What does the parking brake do?

Applies and releases the parking brake when you use the parking brake control

What does the emergency brake do?

Uses parts of the service and parking brake system to stop the vehicle in event of a brake system failure.

What does the air compressor do?

Pumps air into the air storage tank reservoirs. Connected to the engine through gears or a belt. May be cooled by air or engine cooling system. May have its own oil supply or use engine oil.

What does the air compressor governor do?

Controls when the air compressor will pump air into the air storage tanks. When it rises to "cut-out" of 125psi it stops the compressor. When it falls to "cut-in" at 100psi it allows it start again.

What do the air storage tanks do?

Used to hold compressed air.

What are the air tank drains?

Used to drain water and oil. Can be manaul or automatic.

What does the alcohol evaporator do?

Puts alcohol into the air system to reduce the risk of ice in air brake valves. Check the container and fill up every day. Daily tank drainage is still necessary.

What does the safety valve do?

Installed in the first tank and protects the tank and rest of system from too much pressure. Set to open at 150psi.

What does the brake pedal do?

Pushing harder applies more air pressure. Letting up releases air pressure and releases the brakes. This lets some compressed air out of the tanks. This must be made up by the air compressor. Pressing and holding unnecessarily can let air out faster than it can be replaced.

What are foundation brakes?

Used at each wheel. Most common type is teh s-cam drum brake.

What do brake drums, shoes, and linings go?

Drums are located on each end of the axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes heat which can be a problem.

What are s-cam brakes?

When braking air pressure pushes the rod out, moving the slack adjuster, and twisting the brake cam shaft. This turns the s-cam which forces the brake shoes away from one another and against the drum.

What are wedge brakes?

The brake chamber push rod pushes a wedge directly between the ends of two brake shoes to force them against the drum. May have one or two chambers.

What are disc brakes?

Instead of an s-cam a power screw is used.

What do supply pressure gauges do?

Tell how much air pressure is in the tanks. If a dual system there will be one for each half of the system.

What does the application pressure gauge do?

How much air pressure you are applying to the brakes. Increasing application pressure to hold the same speed means the brakes are fading.

When does the low pressure warning signal go off?

Before the pressure falls below 60psi. (Or one half the compressor governor cutout pressure on older vehicles).
On large buses its common to work at 80-85psi.

What is the stop light switch?

Turns on brake lights which the air brake system does with an electric switch that works by air pressure.

What is the front brake limiting valve?

Older vehicles (pre-1975) have this on the front brake and a control in the cab. Control is marked "normal" and "slippery". "Slippery" means cutting the normal air pressure to the front brakes by half. Used to reduce the chance of front wheels skidding when slippery. However, they actually reduce the stopping power.

When does front wheel braking work?

Under all conditions. Front wheel skids from braking are unlikely even on ice.

What is are automatic front wheel limiting valves?

Reduce air to the front brakes except when brakes are put on hard (60psi or more application). Not controlled by the driver.

What are spring brakes?

All trucks, truck tractors, and buses must have emergency and parking brakes held on by mechanical force. Spring brakes meet this need. When driving the springs are held back by air pressure, if air pressure is removed the springs put on the brakes. A parking brake control lets the driver let the air out of the spring brakes.

When do spring brakes come on?

Tractor and straight truck come on when air pressure drops to 20-45psi (typically 20-30psi). When the low air pressure warning light comes on stop the vehicle right away while you can control the brakes.

What does the parking brake look like?

It is a diamond-shaped, yellow, push-pull control knob. You pull it out to the put the parking brake on, push in to release.

What should you never do when spring brakes are on?

Never push the brake pedal down. The brakes can be damaged by the combined forces of the springs and air pressure.

What are the modulating control valves?

A control handle on the dash that can apply the spring brakes gradually. Works so you can control the spring brakes if the service brakes fail.

What are dual parking control valves?

A seperate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency.

Describe dual air brake systems?

Two seperate air brake systems which uses a single set of brake controls. Usually one for the rear axles, and other for the front. First system is "primary".

What do you need to do before driving a vehicle with a dual air brake system?

Allow time for the air compressor to build up a min of 100psi in both systems. Watch the low air pressure warning light. It should shut off when both systems are greater than 60psi.

When should the warning light come on in a dual air brake system?

Before the air pressure drops to 60psi in either system. If it happens, stop right away. Have the system fixed.

What additional check is required in step 2 of inspection?

Check air compressor drive belt.

How do you check the manual slack adjusters on s-cam brakes?

- Check manual slack adjusters on s-cam brakes. Park on level ground and chock the wheels, turn off the parking brake. If it moves more than 1" where the push rod attaches it needs adjustment.
*Automatic adjusters should not have to be manually adjusted. It is probably a mechanical problem and adjusting masks the problem.
- Check brake drums (or discs), linings, and hoses. No cracks longer than half the width of the friction area. Linings must not be loose or soaked with oil.

How do you test for air leakage rate?

- Test air leakage rate. With a fully-charged engine (125psi), turn off the engine, release the service brake, and time the psi drop. It should be <2psi in 1 minute for singles, <3psi in combos. Then apply 90psi more w/t/brake pedal. After that if it falls >3psi in 1 minute for singles or >4psi in combos the air loss rate is too much.

How do you test low pressure warning signal?

Shut off the engine when there is enough psi that the low pressure signal is not on. Turn the electrial power on, and step on/off the brake pedal to reduce psi. The signal must come on before the pressure drops to 60psi.

How do you test that the spring brakes come on automatically?

Chock the wheels, release the parking brake when enough psi to do so, and shut the engine off. Step on/off the brake pedal and the "parking brake" knob should pop out when the psi drops to 20-40.

How do you check the rate of air pressure buildup?

At the operating rpm, the pressure should build from 85-100psi within 45 secs in dual air systems. In single air systems (pre-1975) it should build from 50-90psi within 3 mins with the engine at an idle speed of 600-900rpm

How do you check the air compressor governor cut-in and cut-out pressures?

Pumping the air compressor should start at 100psi and stop at 125psi. Run the engine at fast idle, and it should cut-out the compressor at 125psi. Then step on/off the brake to reduce pressure and it should cut-in and pressure rises.

How do you test the parking brake?

Stop the vehicle, put the brake on, and gently pull against it in a low gear to see if it holds.

How do you test the service brakes?

Wait for normal air pressure, release the parking brake, move forward at about 5mph, and apply the brakes firmly. Any pulling to a side, unusual feel, or delayed stopping is a problem.

How do you use the air brakes in a normal stop?

Push the brake pedal down. Control the pressure to come to a smooth stop. If you have a manual transmission, don't push the clutch in until the rpm is close to idle. When stopped select a gear.

How do you use the air brakes in an emergency stop?

Brake in a way that will keep your vehicle in a straight line and allow you to turn if necessary. You can use controlled or stab braking.

What is controlled braking?

You apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small.

What is stab braking?

Apply the brakes all the way, release when the wheels lock up, as soon as the wheels start rolling apply fully again. If you don't wait for the wheels the vehicle won't straighten out.

What is the stopping distance formula for air brakes?

There is added delay: time required for brakes to work. Hydraulic brakes work instantly, air brakes take a half-sec or more for the air to flow through the lines to the brakes.
The formula is: perception distance + reaction distance + brake lag distance + effective braking distance.

What is the air brake lag distance at 55mph?

On dry pavement it is 32'. Total stoppind distance is over 300'.

What is brake fade?

Braking causes heat, and excessive heat caused by using them too much and not relying on the engine braking effect causes them to fade.

What does brake fade result from?

- Excessive heat causing chemical changes in the brake lining which reduce friction and also causes expansion of the brake drums. Force of contact is reduced.
- Adjustment. Brakes out of adjustment will stop doing there share of work and other brakes can overheat.

What is proper braking technique?

The use of brakes on a downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in low gear do the following:
1. Apply the brakes just enough to feel a slowdown.
2. When speed has been reduced to 5mph below "safe" speed, release the brakes.
3. When speed is at "safe" speed, repeat steps 1 and 2.

What do you do when there is low air pressure?

Stop and safely park the vehicle asap. There might be an air leak and controlled braking is only possible with air in the system. The spring brakes will come on at 20-45psi and it heavy vehicles will take a long time to stop because the spring brakes do not work on all axles. Light vehicles or those on slippery roads may skid out of control.

When should I use the parking brake?

Any time you park.

What are the exceptions to using the parking brake?

1. If the brakes are very hot. Let hot brakes cool before using.
2. If the brakes are very wet in freezing temps. If wet use brakes lightly while driving in low gear to heat and dry them.

When should you use the air tank drains?

If there are not automatic drains, drain the air tanks at the end of each working day to remove moisture and oil.

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