OR CDL Air Brakes cheat sheet

Compressor (engine-driven, gear- or belt-driven) supplies compressed air; it must build enough pressure to keep the system charged, and it runs whenever the engine runs, cycling via the governor.

Governor controls the compressor: it cuts the compressor out at cut-out pressure and back in at cut-in pressure, typically about 100 psi cut-in and 120-125 psi cut-out on many heavy vehicles.

Reservoirs (tanks) store compressed air; a supply/wet tank receives air directly from the compressor and condenses moisture, and dry/front and rear tanks feed the brake circuits.

Air dryer removes moisture and oil vapor after the compressor and before the wet tank; a saturated or frozen dryer is a common cause of moisture and pressure problems.

Foot valve (treadle valve) is the driver's service brake control; it meters air pressure to the brake chambers in proportion to pedal travel.

Brake chamber converts air pressure into mechanical force; a service chamber has one diaphragm for normal braking, while a spring brake chamber adds a mechanically stored spring for parking and emergency.

Slack adjuster transmits pushrod travel to the brake shoe and provides adjustment; excessive pushrod travel means the brakes are out of adjustment and stopping distance increases.

Safety valve on the air tank (typically set near 150 psi) protects the system from overpressure if the governor fails; it is not a normal operating control.

A dual (split) air brake system has two separate circuits, usually front and rear, each with its own reservoir and check valve, so a failure in one circuit does not empty the other.

Check valves prevent air from flowing back out of a reservoir, so a leak in one circuit cannot drain the other circuit's stored air.

The low pressure warning device must activate before the pressure in the service brake system drops below 60 psi (warning light and/or buzzer).

The system must maintain enough pressure to stop; typical minimum is around 100 psi to start driving, and pressure should build from 85 to 100 psi within about 45 seconds at fast idle.

A spring brake (parking brake) applies mechanically when air pressure is released; the pushrod is held back by air pressure, so loss of air applies the brake automatically.

The parking brake control (yellow diamond knob) releases air to apply the spring brakes; pulling it out applies the parking brake, pushing it in releases it when pressure is sufficient.

The tractor protection valve (on combination vehicles) protects the tractor air supply; it closes to keep tractor pressure from being lost through a trailer breakaway or leak.

The trailer supply valve (red knob) controls air to the trailer; it pops out automatically when tractor pressure drops too low, applying the trailer spring brakes.

Check the air compressor drive belt for cracks, glazing, and correct tension; a slipping belt prevents the system from building pressure.

Drain all air tanks daily (or as required) to remove moisture and oil; most tanks have a drain valve at the bottom, and the wet tank should be drained first.

Check the air dryer for a saturated desiccant or a clogged filter; moisture carryover into the tanks indicates the dryer needs service.

Inspect brake chambers for dents, cracks, and loose clamps; the pushrod should move freely and the chamber should not be damaged or leaking.

Check slack adjusters and pushrod travel; with the brakes applied, pushrod travel beyond the manufacturer's limit indicates the brakes need adjustment.

Check brake linings and drums for wear, cracks, and contamination by oil or grease; contaminated linings reduce friction and stopping power.

Inspect all air lines and hoses for chafing, cracks, and leaks, and check that gladhands and couplers are sealed and secure.

Test the low pressure warning and spring brake operation as part of the inspection, not just the service brakes; these safety features must work before driving.

Static leak test: with the engine off and the system fully charged, release the parking brake and hold the service brake; pressure should not drop more than about 3 psi in one minute for a single vehicle (check the manufacturer's specification).

Applied leak test: with the system charged and the engine off, apply and hold the service brake; a steady pressure drop indicates a leak in the service brake system.

Low pressure warning test: with the engine off, pump the brake pedal to reduce pressure; the warning light and buzzer must come on before pressure drops below 60 psi.

Spring brake test: with the system charged, pull the parking brake knob; the spring brakes should apply. Push the knob in with sufficient pressure and they should release.

Low pressure spring brake test: pump the brake pedal down until the parking brake knob pops out automatically, indicating the spring brakes applied due to low pressure.

Governor check: build pressure and observe cut-out and cut-in; the compressor should cut out near the specified maximum and cut back in near the specified minimum.

Build-up check: with the system at low pressure, run the engine at fast idle; pressure should build from 85 to 100 psi within about 45 seconds (check the manufacturer's spec).

If any check fails, the vehicle must not be driven until the fault is repaired; an air brake fault is a safety defect, not a minor issue.

Air brakes are not applied instantly; there is a lag while air travels through the lines, so plan stopping distance and begin braking earlier than with hydraulic brakes.

Use steady, progressive pressure on the treadle valve rather than pumping; pumping wastes air and can cause pressure loss on long downgrades.

On long or steep downgrades, select a low gear before descending and use engine braking; do not rely on the service brakes alone to control speed.

Use the brakes in short, firm applications to slow the vehicle, then release to let them cool; continuous light drag builds heat and can cause fade.

Watch the air pressure gauge continuously on downgrades; if pressure drops, stop and find the cause before continuing.

Do not fan or pump the brakes on a downgrade; each application uses stored air, and repeated use can exhaust the supply faster than the compressor can recover it.

Stopping distance increases with speed, load, and brake temperature; maintain a following distance that allows for the air brake lag and the vehicle's weight.

If the low pressure warning sounds while driving, stop as soon as it is safe; the spring brakes will apply automatically when pressure falls far enough.

If service brakes fail, the spring brakes will apply automatically when air pressure drops low enough; this is the designed fail-safe, not a separate system to activate.

Use the parking brake control (yellow knob) to apply the spring brakes deliberately in an emergency, but apply it gradually; sudden application can lock the wheels and cause a skid.

If a brake chamber or line fails and pressure is lost, the tractor protection valve and check valves isolate the failed circuit so the remaining circuit can still stop the vehicle.

Downshift to a lower gear to use engine braking and help slow the vehicle if the service brakes are weak or fading.

If brakes overheat and fade, release the brakes and let them cool; do not continue to apply them, as heat reduces friction and can damage linings and drums.

If the vehicle cannot be stopped, look for an escape route such as an uphill grade, a runaway truck ramp, or a soft shoulder, and steer toward it while warning others.

After any brake failure or emergency stop, do not resume driving until the cause is found and repaired; the vehicle is not safe to operate with a known brake defect.

Know the warning signs: a low pressure buzzer, a popping parking brake knob, or a pressure gauge that will not build all indicate a brake system problem requiring immediate action.