Pennsylvania CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system by drawing in outside air and forcing it into the storage tanks, typically through a governor-controlled cycle.

Valve device that monitors system pressure and signals the compressor to start pumping at a cut-in pressure and to stop at a cut-out pressure, usually around 100 and 125 psi.

The upper pressure setting, commonly about 125 psi, at which the governor unloads the compressor so it stops building pressure and the system holds steady.

The lower pressure setting, commonly about 100 psi, at which the governor lets the compressor resume pumping air into the tanks to restore normal operating pressure.

Sealed reservoir that holds compressed air for brake application, giving the system reserve capacity so brakes still work for a time after the compressor stops.

Manual or automatic valve at the bottom of an air tank used to release accumulated moisture and oil, which must be drained daily to prevent corrosion and freeze-ups.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, reducing the chance of ice blocking valves and lines in cold weather.

Filter unit that removes moisture and contaminants from compressed air before it reaches the tanks, helping protect valves and lines from corrosion and winter icing.

Pressure-relief valve on the air tank that opens automatically at a set pressure, usually about 150 psi, to protect the system from over-pressurization.

Treadle valve the driver presses to apply the service brakes; the amount of pressure applied varies with how far the pedal is pushed down.

Link between the brake chamber push rod and the brake shoe that converts push-rod stroke into shoe movement; its angle and adjustment affect braking force and stroke length.

Cylinder containing a diaphragm that converts air pressure into mechanical force, pushing a rod that applies the foundation brake at the wheel.

Rod driven outward by the brake chamber diaphragm that transfers force to the slack adjuster; excess travel signals worn linings or out-of-adjustment brakes.

System with two separate air circuits, a primary and a secondary, so a failure or leak in one circuit still leaves the other able to apply some braking force.

One of the two independent air circuits, typically supplying the rear service brakes; a leak here should not deplete the other circuit's air supply.

The second independent air circuit, typically supplying the front service brakes; it retains its own air supply if the primary circuit fails.

The normal foot-operated braking system used for routine stopping and slowing, applying air pressure to the brake chambers only while the pedal is held.

Spring-applied, mechanically held brake that keeps a parked vehicle from rolling; it is released by air pressure and engages when air is removed.

Brake chamber with a heavy spring that applies the brake when air pressure is released, providing automatic emergency and parking braking if air pressure is lost.

Braking system that applies automatically when air pressure drops too low, using spring brakes; also available through the parking brake control in an emergency.

Dash valve, usually a yellow diamond-shaped knob, that releases air to set the spring brakes and admits air to release them for driving.

Visual and audible alert, such as a red light and buzzer, that activates when system pressure falls below a safe level, typically around 60 psi or the manufacturer's setting.

Dash instrument showing air pressure in the brake system; dual systems have separate gauges or a dual gauge for the primary and secondary circuits.

Gauge showing how much air pressure is being applied to the brakes at the moment, helping the driver judge braking effort on grades and slippery surfaces.

Inspection step with the engine off and brakes released, checking that air pressure does not drop faster than the allowed rate, indicating leaks in the system.

Inspection step with the brakes fully applied and held, checking that pressure loss stays within the allowed limit and that no audible leaks are present.

Escape of compressed air through a loose fitting, cracked line, worn seal, or damaged valve; it lowers pressure and can prevent brakes from releasing or applying properly.

Check that the compressor raises pressure from a low reading to the governor cut-out within a reasonable time, showing the compressor and governor are working.

Required walk-around check before driving that includes draining tanks, testing warning devices, checking brake adjustment and leaks, and confirming the parking brake holds.

Inspection of push-rod travel with brakes applied; too much stroke means the brakes are out of adjustment and stopping power is reduced.

Loss of braking power from overheated drums and linings, often after repeated hard stops or long downgrades; air brakes can fade and require lower gears to control speed.

Technique of braking firmly to reduce speed about 5 mph below the safe downgrade speed, then releasing, repeated as needed to avoid overheating the brakes.

Auxiliary slowing device such as an exhaust or compression brake that reduces reliance on the service brakes on long grades, helping prevent brake fade.

Repeated on-off brake applications that waste air and can overheat brakes; drivers should instead use steady pressure or snub braking on downgrades.

The time needed for the compressor to raise system pressure from a low point to normal operating pressure; slow build-up points to a weak compressor or a leak.

Manual bolt used to mechanically compress and release a spring brake when air pressure is unavailable, allowing a vehicle to be moved for repair.

Block placed against a wheel to prevent rolling when parking, especially needed when the spring brakes are released or the vehicle is on a slope.

Parking practice of setting the parking brake, placing the transmission in a low gear or park, and chocking wheels so the vehicle cannot roll downhill.