Illinois CDL Air Brakes cheat sheet

Compressor pumps air into the storage tanks; the governor controls it, cutting it out near 125 psi and back in near 100 psi.

Safety valve on the supply tank vents at about 150 psi to protect the system from over-pressurization if the governor fails.

Air storage tanks hold compressed air; manual drain cocks at the bottom must be opened to remove water and oil.

Alcohol evaporator or air dryer injects alcohol or removes moisture to prevent water from freezing in lines and valves during cold weather.

Foot valve (brake pedal) applies air pressure to the brakes; releasing the pedal exhausts the air and releases the brakes.

Slack adjusters connect the push rod to the brake chamber and allow adjustment of brake shoe travel.

Brake chambers convert air pressure into mechanical force; a spring (emergency) brake applies automatically when air pressure is lost.

The low air pressure warning must activate before pressure drops below 60 psi, and a tractor protection valve protects the trailer air supply.

A dual air system has two separate circuits (primary and secondary), each with its own tank, so a failure in one circuit does not disable the other.

The primary circuit typically operates the rear (drive) axle brakes; the secondary circuit typically operates the front (steering) axle brakes.

The system must retain at least 100 psi in the remaining circuit after a single-circuit failure so the vehicle can still be stopped.

A gauge shows pressure in both circuits; a rapid drop in one gauge indicates a leak or failure in that circuit.

The tractor protection valve closes automatically when trailer air pressure drops to about 20-45 psi, preventing tractor air loss.

The trailer air supply (red) knob and parking brake (yellow) knob are pulled out to set the spring brakes; pushing in charges and releases them.

A leak in one circuit causes that circuit's gauge to fall faster than the other; the vehicle can still be stopped with the remaining circuit.

The front brake limiting valve reduces front braking force in slippery conditions on some older vehicles; modern systems use automatic limiting.

Check that the warning light and buzzer activate when the key is turned on and air pressure is below 60 psi; they must turn off above 60 psi.

With the engine off and tanks full, release the parking brake and watch the gauges: a pressure drop of more than 3 psi in one minute (or 4 psi with engine running) indicates a leak.

With the engine running, build pressure to the governor cut-out (about 125 psi); the compressor should stop pumping.

With the engine off, apply the brakes fully and hold for one minute; a drop of more than 3 psi signals a leak in the applied system.

Test the spring brakes: with the engine off, release the parking brake and watch for the brakes to set automatically as pressure drops below about 45 psi.

Check slack adjuster travel and brake shoe/pad thickness; excessive push rod travel means brakes need adjustment.

Drain air tanks daily to remove moisture; check the air dryer and alcohol evaporator function in cold weather.

Test the safety valve by pulling the ring; it should release air and reseat, confirming it is not stuck.

Air brakes have a lag: allow time for pressure to build before the brakes apply, and for air to exhaust before they release.

Brake early and gradually; do not fan or pump the brakes, which wastes air and reduces stopping ability.

Use the braking effect of the engine (downshift) on long downgrades to avoid overheating the brakes.

Do not ride the brakes; continuous application heats drums and causes brake fade.

When stopping, apply steady pressure and hold the pedal; the spring brakes set automatically when the parking brake knob is pulled.

Before driving, check that air pressure is at a safe operating level (at least 100 psi) and that the low pressure warning is off.

On slippery surfaces, use light, steady pressure and avoid sudden braking that can cause skids.

After the vehicle is parked, set the parking brake and chock wheels if necessary; never rely on air pressure alone to hold the vehicle.

Brake fade occurs when drums and linings overheat from continuous braking, reducing friction and stopping power.

If brakes fade on a downgrade, downshift, use engine braking, and apply the brakes firmly once they cool rather than pumping.

If air pressure drops and the low pressure warning sounds, stop as soon as safely possible and investigate the leak.

If the spring brakes apply automatically due to low pressure, the vehicle will stop; do not attempt to drive until pressure is restored.

If a brake line or chamber fails, the dual system allows stopping with the remaining circuit; use the emergency/parking brake as a backup.

In a runaway situation, look for an escape ramp or use engine braking and downshifting to slow the vehicle.

Do not use the parking brake as a service brake at speed; it can cause a skid or loss of control.

After any emergency stop, inspect the brake system for leaks and damage before continuing.