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Flashcards in Air Brakes Deck (45):

Air Compressor (4)

pumps air into the air storage tanks (reservoirs).

connected to the engine through gears or a V-belt

may be air cooled or may be cooled by the engine cooling system

May have its own oil supply


Air Compressor Governor

The governor controls when the air compressor will pump air into the air storage tanks.

Cut out level = 125psi
Cut in level = 100psi


Air Storage Tanks

used to hold compressed air.


Air Tank Drains

A drain at the bottom of the air tank used to drain water and oil.

water can freeze in cold weather and cause brake failure.

Can be automatic or manual

Manual drained once per day


Alcohol Evaporator

puts alcohol into the air system.

This helps reduce the risk of ice in air brake valves and other parts during cold weather.

Ice inside the system can make the brakes stop working.


Safety Valve

installed in the first tank the air compressor pumps air to.

protects the tank and the rest of the system from too much pressure.

set to open at 150 psi


Foundation Brakes

used at each wheel.

The most common type is the S-cam drum brake


Brake drums

located on each end of the vehicleʼs 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 friction, which slows the vehicle (and creates heat).


Brake Fade

when brakes heat up and lose their effectiveness.


S-cam brakes

When you push the brake pedal, air is let into each brake chamber (see Figure 5-2). Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake cam shaft. This turns the S-cam.
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.


Wedge brakes

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 brake shoes.

Wedge-type brakes may be self-adjusting or may require manual adjustment.


Disc brakes

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.


Why is it dangerous to manually adjust automatic slack adjusters?

because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.


Supply Pressure Gauges

tell you how much pressure is in the air tanks.

dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles).


Application Pressure Gauge

tells you how much air pressure you are applying to the brakes.

harder you press the brake pedal, the higher the pressure

Increasing application pressure to hold the same speed means the brakes are fading.
May also mean brakes are out of adjustment, air leak or mechanical problem.

Slowdown and use a lower gear

Most combination vehicles have this


Low Air-Pressure Gauge

A low air pressure warning signal is required on all vehicles with air brakes.

A visual warning signal must come on before the air pressure in the tanks falls below 60 psi.

Red light or buzzer.

wig wag device must come into view when pressure drops below 60 psi.


Stop Light Switch

an electric switch that works by air pressure.

The switch turns on the brake lights when you put on the air brakes.


Front wheel skids

Tests show that front-wheel skids from braking are not likely even on ice.


front-wheel limiting valves

reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure).

These valves cannot be controlled by the driver.


Spring Brakes

are required on all vehicles using an air brake system.

powerful springs are held back by air pressure.

If the air pressure is removed, the springs put on the brakes.

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.

A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.

spring brakes will come on fully when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi).

Low air pressure warning will activate first. Don’t wait.

Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure.


Parking Brake Controls

controlled using a diamond-shaped, yellow, push-pull control knob.

You pull the knob out to put the parking brakes (spring brakes) on, and push it in to release them.

Every time you are stopped!


Modulating control valves

control handle on the dash board may be used to apply the spring brakes gradually.

control the spring brakes if the service brakes fail.


Dual parking control valves

When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank that can be used to release the spring brakes so you can move the vehicle in an emergency.

Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out.


Dual Air Brake Systems

has two separate air brake systems that use a single set of brake controls.

The first system is called the primary system. The other is called the secondary system.

One system operates the regular brakes on the rear axles and the other operates the regular brakes on the front axle and possibly one rear axle.

Both systems supply air to the trailer if there is one.

air system should be at a minimum of 100 psi before you begin driving.

A fully charged system is approximately 120 psi.

The low air warning device must activate before air pressure reaches 60 psi and may not turn off until air pressure rises above 60 psi. On either system.


What should you do if the low air warning device comes on?

stop right away and safely park the vehicle.

If one air system is very low on pressure, either the front or the rear brakes will not be operating fully.

This means it will take you longer to stop.


Step 2: Engine Compartment Checks

Check air compressor drive belt if compressor is belt driven. - condition and tightness


Air Brake inspection: Slack adjusters

STEP 5: Walkaround Inspection

Park on level ground

chock the wheels to prevent the vehicle from moving.

Turn off the parking brakes so you can move the slack adjusters.

pull hard on each slack adjuster that you can get to.

If a slack adjuster should not move more than about one inch where the push rod attaches.

-too much brake slack can make vehicles very hard to stop


Air brake inspection:

brake drums or discs, linings and hoses

Brake drums or discs must not have cracks longer than one-half the width of the friction area

Linings are free of oil and grease. Must not be too thin.

Make sure air hoses are not cut or worn due to rubbing.


What are slack adjusters?

An adjustable member that transmits brake application force and permits compensation for lining wear


Step 7: final air brake check

Test low pressure warning signal

Do the following checks instead of the hydraulic brake check

Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on.

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 the tank with the lowest air pressure in dual air systems.

* warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system and an increase in distance to stop with a dual system.


Step 7: final air brake check

Check that spring brakes come on automatically

Chock the wheels,
release the parking brakes when you have enough air pressure to do it

shut the engine off.

Step on and off the brake pedal to reduce the air tank pressure.

The “parking brake” knob should pop out when the air pressure falls: usually between 20 and 45 psi.

This causes the spring brakes to come on.


Step 7: final air brake check

Check rate of air pressure buildup

When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems

single air systems (pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

if the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe.

air pressure does not build up fast enough, the pressure may drop too low during driving, requiring an emergency stop.


Step 7: final air brake check

Test air leakage rate

With a fully-charged air system (typically 125 psi)
turn off the engine
release the parking brake
time the air pressure drop.

The loss rate should be less than 2 psi in 1 minute for single vehicles
less than 3 psi in 1 minute for combination vehicles.

Then apply 90 psi or more with the brake pedal.
After the initial pressure drop, the air pressure should not fall more than
3 psi in 1 minute for single vehicles
4 psi for combination vehicles

Check for air leaks, and repair before driving the vehicle.

Otherwise, you could lose your brakes while driving.


Step 7: final air brake check

air compressor governor cut-in and cut-out pressures

air compressor should start at about 100 psi and stop at about 125 psi

Run the engine at a fast idle.
The air governor should cut out the air compressor at about the manufacturerʼs specified pressure (cut out 125 PSI)
The air pressure shown by your gauge(s) will stop rising.

With the engine idling, step on and off the brake to reduce the air tank pressure.
The compressor should cut in at about the manufacturerʼs specified pressure. (Cut in 100)
The pressure should begin to rise.

governor that does not work right may not keep enough air pressure for safe driving.


Step 7: final air brake check

Test parking brake

Stop the vehicle
put the parking brake on, gently pull against it in a low gear to test that the parking brake will hold.


Step 7: final air brake check

Test service brakes

Wait for normal air pressure
release the parking brake
move the vehicle forward slowly (about 5 mph)
apply the brakes firmly using the brake pedal.

Note any vehicle “pulling” to one side, unusual feel or delayed stopping action.


Controlled braking

apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.


Stab braking

Use only on vehicles without anti-lock systems.

Apply the brake all the way.

Release the brakes when the wheels lock up.

As soon as the wheels start rolling, put on the brakes fully again.

It can take up to 1 second for the wheels to start rolling after you release the brakes.

If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.



yellow light near the driverʼs rear side of the vehicle

yellow malfunction lamp on the instrument panel will light up, briefly


Air brake lag

there is a lag between the time you depress the brake pedal and when the brakes actually engage.

the delay can be 1/2 second or more.


Air brake stopping distance

Perception Distance
+ Reaction Distance
+ Brake Lag Distance
+ Effective Braking Distance
= Total Stopping Distance

The air brake lag distance at 55 mph on dry pavement adds about 32 feet.

Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet.


Brake fade

Excessive use of the service brakes results in overheating and leads to brake fade.

Brake fade also is affected by adjustment.

Each brake must do its equal share


Braking technique

use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine.

Apply the brakes just hard enough to feel a definite slowdown.

When your speed has been reduced to approximately 5 mph below your “safe” speed, release the brakes. (This brake application should last about 3 seconds.)

When your speed has increased to your “safe” speed, repeat the first two steps.

For example, if your “safe” speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.


Low air pressure

low air pressure warning device will activate? If you said before 60 psi

Controlled braking is possible only while enough air remains in the air tanks.

The spring brakes will come on when the air pressure drops into the 20 to 45 psi range.

A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles.

Lightly loaded vehicles or vehicles on slippery roads may skid out of control


Parking brake

Any time you park, use the parking brakes

The control will be a yellow, diamond-shaped knob labeled “parking brakes”

Do not use the parking brakes if the brakes are very hot (i.e., from just coming down a steep grade), or if the brakes are very wet in freezing temperatures.

If the brakes are used when very hot, they can be damaged by the heat.
Let brake cool before applying

If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move.
If wet, dry brakes by lightly applying pressure when in a low gear

Use wheel chocks to hold the vehicle.