CDL - AIR BRAKES - (Commercial Driver's License Manual - 2005 CDL Testing System)

Air brakes are really three different braking systems:

The service brake, The parking brake, The emergency brake system

The air compressor governor controls:

When air is pumped into the air 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.

Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. Be sure that you drain the air tanks completely

At the end of your last shift or end of duty day

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps

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

A safety relief valve is 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. The valve is usually set to open at

150 psi. If the safety valve releases air, something is wrong.

The application pressure gauge shows how much air pressure you:

are applying to the brakes

Increasing application pressure to hold the same speed means the brakes are fading. The need for increased pressure can also be

brakes out of adjustment, air leaks, or mechanical problems

A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below

60 psi

On large buses it is common for the low-pressure warning devices to signal at

80-85 psi

The control is usually marked "normal" and "slippery." When you put the control in the "slippery" position, the limiting valve cuts the "normal" air pressure to the front brakes by

half

Limiting valves were used to reduce the chance of

front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle.

Front wheel braking is good under

all conditions.

Tests have shown front wheel skids from braking are not likely even

on ice

Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of

20 to 45 psi

When the low air pressure warning light and buzzer first come on,

bring the vehicle to a safe stop right away, while you can still control the brakes.

you put on the parking brakes using a

diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes

a control handle on the dashboard may be used to apply the spring brakes gradually.

This is called a modulating valve. It is spring-loaded so you have a feel for the braking action. They work this way so you can control the spring brakes if the service brakes fail.

Dual Parking Control Valves.

main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, 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.

ABS is an addition to your normal brakes. It does not decrease or increase your

normal braking capability

normal braking capability Each system

has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle

Dual Air Brake: The first system is called the "primary" system. The other is called the

"secondary" system.

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.

The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should

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.

Check Slack Adjusters on S-cam Brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes so you can move the slack adjusters. Use gloves and

pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.

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

Pumping by the air compressor should start at about 100 psi and stop at about

125 psi.

turn off the engine, release the parking brake (push in); and time the air pressure drops. 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.

fully apply the foot brake. Hold the foot brake for one minute. Check the air gauge to see if the air pressure drops more than

three pounds in one minute (single vehicle) or four pounds in one minute (combination vehicle).

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.

In 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 rpms

Test Service Brakes. Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about five mph)

apply the brakes firmly using the brake pedal. Note any vehicle "pulling" to one side, unusual feel, or delayed stopping action.

Brake the ____________ regardless of whether you have ABS on the tractor, the trailer, or both.

same way

You should brake in a way that will keep your vehicle in a

straight line and allow you to turn if it becomes necessary.

Controlled Braking.

you apply the brakes as hard as you can without locking the wheels.

Stab Braking.

Apply your brakes all the way. Release brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again.

With air brakes there is an added delay - "Brake Lag". This is the time required for the brakes to work after the brake pedal is pushed.

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

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

32 feet

at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over

450ft

brakes can fade or fail from excessive heat caused by using them too much and not relying on the

engine braking effect

Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction, and also causing expansion of the

brake drums

Remember. The use of brakes on a long and/or steep downgrade is only a

supplement to the braking effect of the engine.

When your speed has been reduced to approximately five mph below your

"safe" speed

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.

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 when the spring brakes come on

Don't use the parking brakes if the brakes are very

hot or if the brakes are very wet in freezing temperatures

If the brakes are wet, use the brakes lightly while

driving in a low gear to heat and dry them.

drain your air tanks at the

end of each working day to remove moisture and oil

Never leave your vehicle

unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage