Georgia CDL Air Brakes Test key terms

Engine-driven pump that builds and maintains air pressure in the brake system, typically governed to cut out around 120-125 psi and cut in around 100 psi.

Valve that senses reservoir pressure and signals the compressor to stop pumping at the cut-out pressure and resume at the cut-in pressure.

The maximum reservoir pressure at which the governor unloads the compressor so it stops building air, commonly about 120 to 125 psi.

The lower reservoir pressure at which the governor lets the compressor resume pumping, commonly about 100 psi.

Device between compressor and reservoirs that removes moisture and oil from compressed air to prevent freezing and corrosion in valves and lines.

Storage tank that holds compressed air for braking; a vehicle has at least one supply tank and separate tanks feeding the front and rear brake circuits.

The first reservoir after the compressor and air dryer; it collects moisture and is drained most often during inspection.

Manual or automatic valve at the bottom of a reservoir used to blow out accumulated water and oil; manual valves are opened during pre-trip inspection.

Spring-loaded relief valve on the supply reservoir that opens automatically to release air if pressure rises above a safe limit, usually near 150 psi.

The brake pedal assembly that meters air from the reservoirs to the brake chambers in proportion to how far the driver pushes it.

Cylinder containing a diaphragm and push rod that converts air pressure into mechanical force to apply the foundation brakes.

Rod moved by the brake chamber diaphragm that transfers force to the slack adjuster; its free stroke is measured to judge brake adjustment.

Lever on the brake camshaft that links the push rod to the foundation brake and takes up clearance as linings wear.

Condition in which push rod travel stays within the limit marked on the chamber; excessive travel means the brakes are out of adjustment and unsafe.

Drum brake in which the push rod rotates a camshaft whose cam spreads the brake shoes against the drum.

Rotating metal drum bolted to the wheel hub that the brake shoes press against to slow the vehicle; it must be free of cracks and heavy heat damage.

Friction material bonded to the brake shoes or pads; it must be thick enough and free of oil, grease, and cracks to stop the vehicle.

Two separate air circuits, usually front and rear, fed from a common supply so that a failure in one circuit still leaves braking ability in the other.

The air circuit that normally operates the rear brakes; it is charged first and protected so a leak elsewhere does not drain it.

The air circuit that normally operates the front brakes; it stays charged if the primary circuit fails, giving limited stopping ability.

The portion of the system that stores and delivers compressed air to the other circuits, including the compressor, dryer, and wet tank.

One-way valve that allows air to flow in only one direction, keeping a failed circuit or lost supply from draining the remaining reservoirs.

Warning light and buzzer that activate when reservoir pressure drops below a set level, typically around 60 psi, alerting the driver before braking is lost.

Pre-trip test in which the engine is off and the pedal is pumped to reduce pressure; the warning must come on before pressure falls below about 60 psi.

Parking and emergency brake that uses a strong spring to apply the brakes when air pressure is released, and air pressure to hold the spring off.

Dash valve, usually yellow diamond-shaped, that releases air from the spring brake chambers to apply the parking brake and admits air to release it.

Feature that prevents service brake air and spring brake spring force from applying at the same time, avoiding damage to brake chambers and linkage.

Check with the engine off, full pressure, and brakes released to see how much pressure is lost per minute; a loss over about 3 psi in one minute indicates a leak.

Check with the engine off and the brake pedal held down to measure pressure loss with the brakes applied; a loss over about 4 psi in one minute signals a leak.

Dash instrument showing reservoir pressure; the driver watches it during tests and while driving to confirm the compressor keeps pressure in the normal range.

Pressure delivered to the brake chambers when the foot valve is depressed; it rises with pedal force and falls as the pedal is released.

Overlap of service brake application and spring brake force on the same wheel, which can overstress chambers and linkage; prevented by anti-compounding valving.

Repeatedly applying and releasing the brakes instead of holding steady pressure, which wastes air and can leave too little pressure to stop.

Loss of stopping power when brakes overheat from prolonged or repeated use, especially on long downgrades; gears should be used to control speed instead.

Technique of braking firmly to reduce speed by about 5 mph, then releasing, repeated on a downgrade so brakes cool between applications.

Controlled stop made when air pressure is lost or brakes fail, using the spring brake and gearing down while keeping the vehicle under control.

The measured drop in reservoir pressure per minute during leak tests; excessive loss means the vehicle must be taken out of service until repaired.

Device that injects alcohol into the air system to lower the freezing point of moisture so valves and lines do not ice up in cold weather.

Time needed for reservoir pressure to rise from cut-in to cut-out with the engine running; unusually long build-up points to a weak compressor or a leak.