California CDL Air Brakes Test cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; the governor controls it by cutting the compressor in at about 100 psi and out at about 125 psi.

The air dryer removes moisture and oil vapor from compressed air before it reaches the storage tanks; most dryers purge automatically when the governor unloads the compressor.

The supply (wet) tank is the first tank after the compressor and dryer; it collects condensation and feeds the other tanks through one-way check valves.

The foot valve (brake valve) is the driver's control: pressing the treadle meters air pressure to the brake chambers in proportion to pedal travel.

The brake chamber converts air pressure into mechanical force; a type 30 chamber has 30 square inches of effective diaphragm area, and pushrod force equals pressure times area.

The slack adjuster multiplies and transfers pushrod force to the S-cam, which spreads the brake shoes against the drum; automatic slack adjusters maintain pushrod stroke automatically.

The relay valve speeds application and release at the rear axle by sending tank air directly to the chambers, using a small control signal from the foot valve.

The low-pressure warning device must activate before pressure drops below 60 psi, and the system must build from 85 to 100 psi within 45 seconds at governed speed.

A dual air brake system has two separate circuits, typically primary (rear brakes) and secondary (front brakes), each with its own reservoir, check valve, and warning light.

Total system pressure is read on a gauge with two needles; each needle shows the pressure available in one circuit, and a circuit failure shows as a needle drop or a red warning lamp.

One-way check valves between tanks prevent a leak in one circuit from draining air from the other circuit or from the supply tank.

The tractor protection valve protects the tractor's air supply: it closes when tractor pressure falls to about 20-45 psi, and the trailer air supply valve (red knob) pops out to apply trailer brakes.

The safety valve on the supply tank opens at about 150 psi to prevent tank rupture if the governor fails to unload the compressor.

Trailer air lines are color-coded: the supply (emergency) line is red, and the service line is blue; the electric cable is separate from both.

The alcohol evaporator or air dryer injects alcohol into the air system to prevent moisture from freezing in lines and valves during cold weather.

A compressor that fails to build pressure or builds too slowly may have a failed governor, a leaking discharge line, a worn compressor, or a clogged air filter.

With the engine off and the system fully charged, release the parking brake and listen: a leak in the applied-brake system is indicated by an audible air loss.

Leak rate limits for a single vehicle: with the engine off, brakes released, pressure loss should be no more than 2 psi in one minute; with brakes applied, no more than 3 psi in one minute.

For a tractor-trailer combination, the limits are 3 psi per minute with brakes released and 4 psi per minute with brakes applied.

Test the low-pressure warning: with the engine running, pump the brake pedal to reduce pressure; the warning light and buzzer must activate before pressure drops below 60 psi.

Test the spring (emergency) brakes: continue pumping until the red trailer air supply knob and the yellow parking brake knob pop out, which should occur between 20 and 45 psi.

Check the buildup rate: with the system at 85 psi, idle the engine and observe the gauge; pressure must rise to 100 psi within 45 seconds.

Check the air compressor drive belt for cracks, glazing, and correct tension, and check that the air dryer purges moisture when the compressor unloads.

Inspect brake chambers for cracks, dents, and a loose or misaligned clamp ring, and check that pushrods and slack adjusters move freely without binding.

Watch the air gauge constantly while driving; normal operating pressure is roughly 100 to 125 psi, and pressure below 100 psi signals a problem to investigate.

Brake with a firm, steady pedal application; do not pump the brakes, because each pump cycle uses stored air and can exhaust the system on a long downgrade.

On a long or steep downgrade, select a low gear before starting down and use engine braking plus light, steady brake applications to hold a safe speed.

Use the highest gear that keeps the engine below its governed rpm and the vehicle at a safe speed; snub braking (short, firm applications) is safer than continuous light drag.

When the engine is cold, idle it briefly to build air pressure before moving; never drive until the low-pressure warning has gone out and pressure is at operating level.

If the low-pressure warning sounds or the gauge falls below 60 psi, stop as soon as it is safe, park, and find the leak before continuing.

Test the brakes after driving through deep water: apply them lightly to heat and dry the linings, and check that stopping power returns.

Use the trailer hand valve only for testing trailer brakes; it must not be used for normal stopping because it does not apply the tractor brakes.

A rapid loss of air pressure is an emergency: downshift, use engine braking, and apply the brakes gently to bring the vehicle to a controlled stop off the roadway.

If a circuit fails, the dual system isolates it; the remaining circuit still provides braking, but stopping distance increases and the vehicle must be taken out of service.

The spring brakes apply automatically when air pressure drops to about 20-45 psi; this is the system's fail-safe feature and cannot be released without air pressure or manual caging.

To cage a spring brake for towing or repair, insert the caging bolt and tighten it fully; caged brakes have no parking or emergency braking force.

If the brake pedal goes to the floor with no resistance, suspect a failed foot valve or a major line rupture; use engine braking, the parking brake, and roadside terrain to stop.

A stuck or dragging brake shows as a hot drum, smoke, or a pull to one side; stop, let it cool, and do not continue driving until the cause is corrected.

If the compressor fails and pressure cannot build, do not continue; the remaining stored air is limited, and the spring brakes will apply when it is exhausted.

An air line that freezes in winter blocks air flow; use an alcohol evaporator or air dryer, and drain tanks daily to limit moisture buildup.

The yellow diamond-shaped parking brake knob controls the spring brakes; pulling it out applies them, and pushing it in releases them only when air pressure is sufficient.

The red octagonal trailer air supply knob controls air to the trailer; pulling it out closes the tractor protection valve and applies the trailer spring brakes.

Always set the parking brake and chock the wheels before leaving the cab; spring brakes can be released by air pressure loss or by caging.

Retarders (engine, exhaust, or driveline) slow the vehicle on downgrades and save service brakes, but they do not replace the service brakes or the parking brake.

Retarders can cause loss of traction on wet, icy, or slippery surfaces; turn the retarder off when the road is slippery or when the drive wheels may lose grip.

ABS prevents wheel lockup during hard braking and helps maintain steering control; it does not shorten stopping distance on all surfaces and does not replace safe following distance.

An ABS warning lamp that stays on after the vehicle starts means the system is disabled; the vehicle still has normal hydraulic or air braking without antilock function.

On vehicles with ABS, brake normally and do not pump the pedal; hold firm pressure and steer, letting the system modulate the brakes.