NM CDL Air Brakes flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-out pressure

The pressure at which the governor stops the compressor from building more air, typically around 120 to 125 psi in a normal system.

Governor cut-in pressure

The pressure at which the governor restarts the compressor after it has cycled off, usually near 100 psi, restoring the working range.

Air dryer purge valve

A valve that releases moisture and contaminants from the air dryer each time the compressor cycles off, keeping the supply air dry.

Supply (wet) tank

The first tank to receive air from the compressor; it collects most moisture and has a safety valve to protect against overpressure.

Foot valve (treadle)

The pedal-operated valve that meters air to the brake chambers; releasing it exhausts air so spring pressure can release the brakes.

Spring brake

A parking and emergency brake held released by air pressure; when air is lost or the yellow knob is pulled, the spring applies the brakes.

Slack adjuster

A linkage that converts pushrod stroke into brake shoe movement; excessive pushrod travel signals the brakes are out of adjustment.

Tractor protection valve

A valve that closes the trailer supply line and prevents air loss from the tractor if the trailer breaks away or pressure falls too low.

Static leak test

With full pressure and engine off, watch the gauge; a loss over 3 psi per minute on a single vehicle indicates a leak needing repair.

Combination leak limits

For a tractor-trailer, acceptable static loss is about 4 psi per minute brakes released and about 6 psi per minute brakes applied.

Low-air warning test

The buzzer and light must activate before pressure drops below 60 psi, warning the driver of insufficient air for safe braking.

Spring brake application point

Spring brakes should fully apply automatically when system pressure falls to roughly 20 to 45 psi, providing a fail-safe stop.

Compressor build rate

With the engine at fast idle, pressure should climb from 85 to 100 psi within about 45 seconds, showing the compressor is working.

Pushrod stroke check

Measure pushrod travel with brakes applied; travel beyond the manufacturer limit means the slack adjuster needs adjustment before driving.

Tank draining

Opening the drain cocks on the supply and dry tanks removes condensed moisture that can freeze lines or corrode the system.

Hose and chamber inspection

Look for cracks, chafing, and leaking fittings on brake chambers and hoses; a damaged diaphragm or cut hose causes pressure loss.

Air brake lag

Air brakes apply slightly after the pedal is pressed because air must travel to the chambers, so following distance should be increased.

Progressive braking

Applying steady, increasing pedal pressure instead of pumping; fanning the pedal wastes air and can drop pressure below safe levels.

Downhill braking technique

Select a low gear before descending and use engine braking with light steady brake use, avoiding continuous hard application that overheats brakes.

Holding on a grade

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

Parking procedure

Build full pressure, set the spring parking brake, and chock the wheels on a grade; the spring brake is the primary parking brake.

Wet-weather braking

Brake gently and early in rain or slippery conditions; abrupt air brake application can lock wheels and cause a skid.

Gauge monitoring

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

Stopping technique

Apply the brakes early and smoothly, and at a stop hold firm pedal pressure to keep full air pressure and prevent roll-back.

Low-pressure response

If the warning sounds or pressure falls below 60 psi, stop as soon as safe; spring brakes will apply automatically below about 20 to 45 psi.

Rapid air loss

Often caused by a broken line or failed chamber; pull off the road, set the parking brake, and do not rely on the foot brake.

Unexpected spring brake

If spring brakes apply while moving, keep steering, downshift if possible, and bring the vehicle to a controlled stop without fighting the brakes.

Trailer breakaway

When a trailer separates, the tractor protection valve closes and the trailer spring brakes apply automatically, stopping the trailer.

Compressor failure

With pressure falling and no compressor, brake early and avoid repeated applications so remaining air gets the vehicle safely off the road.

Frozen air line

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

Ruptured diaphragm

A failed brake chamber diaphragm causes continuous air leakage and loss of braking at that wheel; reduce speed carefully and stop for repair.