MA CDL Air Brakes flashcards

32 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. A cut-out far above this range indicates a faulty governor.

Governor cut-in pressure

The pressure at which the governor restarts the compressor after cut-out, typically around 100 psi. A cut-in well below this value signals governor or compressor trouble.

Safety valve

A valve on the supply reservoir that opens around 150 psi to release excess pressure. It protects the system from over-pressurization and is not a normal pressure regulator.

Slack adjuster

A lever that converts brake chamber pushrod stroke into rotation of the brake cam or S-cam. Excessive pushrod stroke means the brakes are out of adjustment.

Spring brake chamber

A chamber with a strong spring that applies the parking brake mechanically when air pressure is released. It releases only when system pressure is above about 45 psi.

Foot valve (treadle)

The graduated brake control pedal. Pushing it further delivers higher pressure to the brake chambers, so braking force increases with treadle travel.

Air dryer function

A device that removes moisture and oil from compressed air before it reaches the reservoirs. A saturated dryer lets water into valves and causes freeze-ups in cold weather.

Low-pressure warning

A buzzer and/or light that must activate before system pressure drops below about 60 psi. It warns the driver that spring brakes will soon apply automatically.

Dual air brake system

Two separate circuits, usually primary for rear brakes and secondary for front brakes, each with its own reservoir. Check valves isolate them so one failure does not drain both.

Tractor protection valve

A valve that closes automatically when tractor supply pressure drops, typically below about 45 psi, preventing air loss if the trailer separates from the tractor.

Trailer supply valve

The red push-pull knob that charges and releases the trailer air supply. It pops out automatically on pressure loss to signal breakaway protection.

Parking brake valve

The yellow push-pull knob that controls the tractor spring brakes. Pulling it out applies the spring brakes; pushing it in releases them when pressure is sufficient.

Check valve in split system

A one-way valve that keeps air from flowing back from one circuit to the other, so a leak in one circuit does not drain the intact circuit.

Front brake limiting valve

A control that reduces front braking force for slippery conditions. It must be in the normal position for dry-road stopping to preserve full braking performance.

Compressor build rate test

With the engine at operating rpm, time the pressure rise from about 85 to 100 psi. Excessive time indicates a weak compressor, slipping belt, or leak.

Applied leak test

With the system charged and engine off, hold the foot valve fully applied for one minute. Pressure loss beyond about 3 psi per minute indicates a leak.

Static leak test

With the system charged and engine off, observe pressure loss with brakes released. Loss beyond about 2 psi per minute indicates a leak requiring repair.

Reservoir draining

Opening the drain cocks on all reservoirs to remove moisture and check for oil. Milky or oily discharge indicates compressor or air dryer problems.

Pushrod stroke limit

The maximum travel of the brake chamber pushrod specified by the manufacturer. A stroke beyond this limit means brakes are out of adjustment and must be serviced.

Spring brake hold test

With the parking brake set, the vehicle must not roll. Failure to hold indicates weak spring brakes or a linkage defect and is an out-of-service condition.

Brake lag

The short delay between applying the treadle and the brakes actually engaging, because air must travel through lines. It requires increased following distance.

Fanning the brakes

Repeatedly pumping the treadle instead of holding steady pressure. On air brakes it wastes air and reduces available braking pressure, so it should be avoided.

Gear selection on grade

Choosing a gear before starting down a grade that holds the vehicle without braking. Shifting down on the grade may be impossible once speed builds.

Brake fade

Loss of stopping power caused by overheated drums and shoes from continuous light brake application on a long downgrade. It is prevented by gearing down and snubbing.

Snub braking

A firm, brief brake application that slows the vehicle, followed by release to let the brakes cool. It is used on downgrades instead of continuous light pressure.

Escape ramp use

A gravel or sand ramp for a vehicle that cannot control speed on a downgrade. Aim for the center and expect a soft, decelerating surface.

Wig-wag brake light

A swinging brake lamp that warns following drivers of aggressive braking. It is required on vehicles built after the applicable rule date and is not a brake control.

Compressor failure symptom

Inability to build pressure to at least 85 psi with the engine running, or an excessively slow build, indicates a worn compressor, slipping belt, or major leak.

Over-pressurization defect

A governor stuck in the cut-in position lets pressure keep rising until the safety valve releases. This is a defect requiring repair before driving.

Saturated air dryer

A dryer whose desiccant can no longer absorb moisture, allowing water into the system. It causes frozen valves in winter and corrosion in valves and reservoirs.

Low-pressure warning while driving

If the buzzer sounds during driving, stop as soon as it is safe. Pressure loss will eventually apply the spring brakes and stop the vehicle on its own.

Out-of-service air brake defects

Examples include an inoperative low-pressure warning, air loss beyond the allowable rate, inability to build 85 psi, and spring brakes that do not apply automatically.