NM CDL Air Brakes cheat sheet

A compressor, driven by the engine, builds air pressure; a governor cuts the compressor out near 120-125 psi and back in near 100 psi, so the normal operating range is roughly that band.

The air dryer removes moisture and oil before air enters the tanks; its purge valve releases collected water and contaminants each time the compressor cycles off.

Supply (wet) tank receives air first and holds the bulk of the moisture; the dry tank feeds the brake circuits; a safety valve on the supply tank protects against compressor or governor failure.

A foot valve (treadle) controls air delivery to the brake chambers through dual circuits; releasing the treadle exhausts air and spring pressure returns the shoes.

The parking/emergency spring brake is held released by air pressure; losing air pressure or pulling the yellow knob exhausts that air and the spring applies the brakes.

Slack adjusters convert pushrod stroke to brake shoe movement; the pushrod should travel about 3/8 to 1/2 inch for a Type 30 chamber before adjustment is needed.

An alcohol evaporator or air dryer protects against winter freeze-up; a low-pressure warning buzzer and light must activate before pressure drops below 60 psi.

A tractor protection valve keeps air from escaping the tractor when the trailer breaks away and closes the trailer supply line when pressure falls too low.

Do a leak test: build full pressure, shut off the engine, release the parking brake, and watch the gauge; a loss of more than 3 psi in one minute (single vehicle) indicates a leak.

For a tractor-trailer combination, the acceptable static leak rate is no more than 4 psi in one minute with the brakes released and no more than 6 psi with them applied.

Check the low-air warning: it must come on before pressure drops below 60 psi, and the spring brakes must fully apply by the time pressure reaches about 20-45 psi.

Test the compressor build rate: with the engine at fast idle, pressure should rise from 85 to 100 psi within 45 seconds in a single vehicle.

Inspect slack adjusters and pushrods with the brakes released and again applied; excessive pushrod stroke means the brakes are out of adjustment and must be fixed before driving.

Drain air tanks daily (or per manufacturer guidance) at the drain cocks on the supply and dry tanks to remove moisture; never drain while the system is under full pressure without eye protection.

Check brake chambers and hoses for cracks, chafing, and air leaks at fittings; a leaking diaphragm or a cut hose will cause slow pressure loss and weak braking.

Verify the air dryer and alcohol evaporator are functioning and that the purge valve cycles; a failed dryer lets moisture reach the tanks and can freeze lines in cold weather.

Air brakes are not instantaneous: allow a lag time before the brakes apply, so increase following distance compared with hydraulic-braked vehicles.

Use controlled, progressive braking and avoid fanning or pumping the pedal, which wastes air and can drop pressure below safe levels.

On long downgrades, select a low gear before the descent and use engine braking plus light, steady brake application to control speed rather than riding the brakes.

Never hold the vehicle on a hill with the foot valve; use the parking (spring) brake or a wheel chock to prevent overheating and air loss.

Before parking, allow the system to build full pressure, set the parking brake, and on a grade chock the wheels; the spring brake is the primary parking brake.

When stopping, apply the brakes early and smoothly; at a stop, hold the pedal firmly to keep full pressure and prevent roll-back.

In wet or slippery conditions, brake gently and early; abrupt air brake application can lock wheels and cause a skid, especially on empty trailers.

Check the gauge regularly while driving; if pressure falls below the safe range, stop and find the leak before continuing.

If the low-pressure warning sounds or the gauge falls below 60 psi, stop as soon as it is safe and do not continue until the leak is fixed; spring brakes will apply automatically below about 20-45 psi.

A rapid loss of air is often a broken line or failed chamber; pull off the road, set the parking brake, and do not rely on the foot brake to hold the vehicle.

If the spring brakes apply unexpectedly while moving, keep steering, downshift if possible, and bring the vehicle to a controlled stop; do not fight the brakes.

The tractor protection valve closes the trailer supply when pressure drops too low, so a trailer breakaway applies the trailer spring brakes automatically.

If the compressor fails and pressure is dropping, use the remaining air wisely: brake early, avoid repeated applications, and get off the road before pressure is exhausted.

A frozen air line can be thawed with an alcohol evaporator or by warming the line; never use an open flame, and drain tanks to reduce moisture.

If a brake chamber diaphragm ruptures, that wheel loses braking and air leaks continuously; reduce speed carefully and stop for repair.

Know the emergency stop drill: signal, steer to the shoulder, apply the parking brake, and place warning devices before exiting the vehicle.