LA CDL Air Brakes cheat sheet

A compressor, driven by the engine, builds air pressure and stores it in reservoirs; the governor unloads the compressor at the cut-out pressure (about 120-125 psi) and reloads it at cut-in (about 100 psi).

The supply (wet) reservoir is the first tank after the compressor; it collects condensed moisture and feeds the remaining reservoirs through the one-way check valves.

A one-way check valve allows air to flow into a reservoir but prevents it from bleeding back, so the loss of the compressor or the supply tank does not empty the other tanks.

The foot valve (treadle valve) in the cab is a graduated control: it delivers air pressure to the brake chambers in proportion to how far the pedal is pressed, not simply on or off.

In a dual-circuit system, the two separate circuits (for example front and rear axles) share one foot valve but have separate reservoirs; a failure in one circuit leaves the other circuit pressurized.

A slack adjuster converts the pushrod stroke of the brake chamber into rotation of the brake camshaft (S-cam) or the wedge, and it is the part that compensates for lining wear.

A spring brake (emergency/parking chamber) contains a strong spring that applies the brakes when air pressure is released; the spring is held off by air, so loss of air applies the brakes.

A low-pressure warning switch triggers a buzzer and/or light when the system pressure falls below about 60 psi, and a pressure gauge shows the pressure in each circuit.

With the engine off and the system fully charged, the static leak test: hold the pedal fully down for one minute; a loss of more than 3 psi in a single-circuit system, or more than 4 psi in a dual-circuit system, fails the test.

The applied leak test: charge the system, release the parking brake, hold the foot brake fully applied for one minute; a drop of more than 3 psi (single) or 4 psi (dual) indicates a leak.

The low-pressure warning test: with the engine off, fan the brakes to reduce pressure; the warning buzzer/light must activate by the time pressure falls to about 60 psi.

The spring brake (emergency) test: with pressure below about 60 psi, the parking brake valve must pop out (or the spring brakes must apply) as pressure continues to drop; a properly working system applies the spring brakes before pressure reaches zero.

The compressor build-up test: at fast idle, pressure should rise from about 85 psi to 100 psi within about 45 seconds in a typical system (check the manufacturer's specification).

The governor cut-out and cut-in test: the compressor should unload at about 120-125 psi and reload at about 100 psi; record both readings.

Visual pre-trip: check the slack adjuster pushrod travel against the limit marked on the chamber (typically no more than about 1 inch for a type 30 chamber, and always within the manufacturer's limit), check the lining thickness and drum condition, and check the hoses for cracks or chafing.

Drain the supply and service reservoirs at the end of the day to remove moisture and oil; a large amount of water or oil in the drain indicates a compressor or air dryer problem.

Air brakes have a lag: pressure takes time to travel from the foot valve to the chambers, so the stopping distance is longer than with hydraulic brakes; allow extra following distance.

Brake fade from overheating can be reduced by gearing down and using engine braking on long downgrades, and by applying the brakes firmly and briefly rather than lightly and continuously.

On a long steep downgrade, select a gear low enough that the engine and the brakes together hold the speed; do not rely on the foot brake alone.

The parking brake valve (yellow diamond knob) must be pushed in to release the spring brakes and pulled out to apply them; the brakes cannot be released until system pressure is above about 60 psi.

In a dual-circuit system, if one circuit fails, the other still provides braking, but stopping distance increases and the low-pressure warning may activate; stop and check the system.

The air dryer removes moisture and oil from the compressed air before it reaches the reservoirs; a failed dryer lets water into the system, where it can freeze in cold weather and block the valves.

Do not fan (pump) the brakes; each application uses stored air, and repeated fanning can exhaust the reservoirs faster than the compressor can recharge them.

Before driving, check that the pressure is at the governor cut-out, the warning buzzer is off, and the parking brake releases fully when the valve is pushed in.

A low-pressure warning buzzer or light at about 60 psi means the system is losing pressure; stop as soon as it is safe and do not continue driving.

If the spring brakes apply by themselves (the parking brake valve pops out), the system has lost pressure; the vehicle will stop, and it cannot be moved again until pressure is restored.

A rapid loss of pressure accompanied by a loud hiss usually indicates a burst hose, a failed chamber diaphragm, or a broken line; stop and do not attempt to drive.

If the foot brake fails, use the engine brake, downshift through the gears, and apply the parking (spring) brake gradually; a sudden full application of the spring brake at speed can cause a skid or loss of control.

A dragging brake (one wheel hot, pulling to one side, or a burning smell) can overheat and fade; stop, let it cool, and have the cause corrected before continuing.

If the compressor fails, the remaining stored air is limited; use the brakes sparingly and stop at the first safe place.

An air brake system that will not build pressure above about 60 psi cannot release the spring brakes; do not try to drive through the spring brakes, because they will overheat and fail.

Static leak test (engine off, fully charged, parking brake released): hold the foot brake fully applied for one minute; more than 3 psi loss (single circuit) or 4 psi (dual circuit) fails.

Low-pressure warning test: with the engine off, reduce pressure by fanning the brakes; the warning must sound by about 60 psi.

Spring brake test: with pressure below about 60 psi, the parking brake valve must pop out and the spring brakes must apply; if they do not, the system fails the test.

Compressor build-up test: at fast idle, pressure should rise from about 85 to 100 psi in about 45 seconds (check the manufacturer's specification).

Governor test: the compressor should cut out at about 120-125 psi and cut in at about 100 psi; a cut-out that is too high or too low indicates a governor or compressor problem.

A leak that can be heard or felt at a hose, fitting, or chamber is a defect; a soap solution applied to the joint will bubble at the leak point.

Pushrod travel beyond the limit marked on the chamber (or beyond the manufacturer's specification) means the slack adjuster or the foundation brake needs service; do not adjust the slack adjuster to compensate for worn linings.

Drain the reservoirs daily and check the air dryer; water or oil in the drained air indicates that the dryer or the compressor is failing and needs service.