MA CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system reservoirs. It is governed to stop pumping near 120-125 psi and restart near 100 psi so the system always has usable pressure.

Valve on the compressor that monitors system pressure and turns compressor pumping on and off, typically cutting out around 120-125 psi and cutting back in around 100 psi to prevent overpressurization and excessive moisture.

Canister between the compressor and the reservoirs that removes water vapor and oil from compressed air. Moisture left in the system can freeze in cold weather or corrode valves, so the dryer and its purge cycle matter for reliability.

Storage tank that holds compressed air for braking. A tractor-trailer has at least one supply tank and separate tanks for the front and rear service brakes, plus a park/emergency tank, so one circuit can fail without emptying the others.

The first reservoir that compressed air enters after the compressor and dryer. Because most condensation collects here, it has a drain valve and is checked first during pre-trip inspection and daily draining.

Manually or automatically operated valve at the bottom of each reservoir that releases accumulated water and oil. Draining tanks daily, and until only clean air escapes, keeps contaminants out of brake valves and lines.

Spring-loaded relief valve on the air system that opens if pressure climbs above a safe limit, usually around 150 psi, to protect tanks and lines from bursting if the governor fails to cut out the compressor.

Brake system split into two independent circuits, typically one for the front axle and one for the rear axles, each with its own reservoir. A failure in one circuit still leaves partial braking in the other.

The rear-brake circuit in a standard dual system, usually supplied by its own reservoir. Losing primary pressure normally triggers a warning light and buzzer and leaves only front-axle braking, so stopping distance increases greatly.

The front-brake circuit in a standard dual system, with its own supply tank and protection valve. If it fails, rear brakes still work, but steering-axle braking is lost and the vehicle may pull or behave differently under hard braking.

The brake pedal valve the driver applies with the foot. It meters air pressure to the service brakes in proportion to pedal travel, so light pedal pressure gives light braking and full travel gives maximum application.

Cylinder that converts air pressure into mechanical force. In a spring brake chamber, a service side applies during normal braking and a spring side applies automatically when air pressure is lost, providing parking and emergency braking.

Emergency and parking brake mechanism held released by air pressure and applied by a heavy spring when pressure drops or the parking valve is set. It is why a vehicle cannot be moved until system pressure builds.

Rod driven out of the brake chamber by air pressure or spring force to apply the foundation brake. Its stroke length is measured in pre-trip inspection; excessive stroke means adjustment or repair is needed.

Linkage between the push rod and the brake shoe that converts push-rod travel into shoe movement and takes up wear. A loose or improperly adjusted slack adjuster causes long push-rod stroke and weak braking.

Slack adjuster that self-adjusts during normal brake applications to maintain correct push-rod stroke. It still requires inspection; if stroke is too long, it must be repaired rather than manually cranked repeatedly.

Maximum permitted push-rod stroke before brakes are out of adjustment, commonly about 2 inches for type 30 chambers at 90 psi. Exceeding it means the brake must be adjusted or repaired before driving.

Pre-trip test in which a helper holds a full brake application while the driver measures how far each push rod extends. Strokes beyond the adjustment limit indicate brakes that will not apply fully.

Light and buzzer that activate when system pressure falls below a set level, usually about 60 psi. It warns that spring brakes may soon apply automatically and that braking power is nearly gone.

Dashboard instrument showing pressure in the brake circuits, often with separate needles for primary and secondary. The driver watches it to confirm buildup, normal operating range, and that pressure is not falling while driving.

Check that the compressor raises pressure from about 85 to 100 psi within a specified time, commonly 45 seconds at fast idle. Slow buildup points to a weak compressor, leaks, or a restricted line.

Test in which the engine is off, brakes released, and pressure is watched for one minute. A loss of more than about 2 psi in that minute indicates an air leak that must be found and fixed.

Leak check with the engine off and the brake pedal held fully down for one minute. Loss of more than about 3 psi signals a leak in the service brake system that must be repaired before operation.

Soapy-water or listening inspection of lines, fittings, valves, and chambers for escaping air while the system is charged. It locates the source of pressure loss found by the leak-down and applied leakage tests.

Tubing or hose carrying compressed air between compressor, reservoirs, valves, and brake chambers. Cracked, chafed, or loose lines leak pressure and can cause slow brake release or failure to build pressure.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture. It is a cold-weather aid, not a substitute for draining tanks and maintaining the air dryer.

Dash valve, usually yellow and diamond-shaped, that releases air to set or release the spring brakes. Pulling it applies the parking brakes; pushing it in releases them only when system pressure is adequate.

Valve that prevents service brake air pressure and spring brake force from applying at the same time on the same chamber, avoiding excessive mechanical stress and wheel lockup when parking brakes are set during a service application.

Valve that closes the tractor's air supply to the trailer if the trailer breaks away or a major trailer leak occurs, preserving tractor braking and causing the trailer spring brakes to set.

Hose, usually color-coded, that carries air from tractor to trailer reservoirs to charge the trailer system. A disconnected or leaking supply line triggers the tractor protection valve and sets trailer spring brakes.

Dash valve, usually red and diamond-shaped, that lets the driver apply the trailer brakes independently of the foot pedal. It is used to test trailer brake operation and to check for a pull during inspection.

Trailer valve that speeds application and release of trailer brakes by supplying reservoir air directly to the chambers, and that applies the trailer spring brakes if supply pressure is lost.

Repeated on-off brake applications that use air faster than the compressor can replace it. On long downgrades this drains pressure and can trigger spring brake application, so steady controlled braking is preferred.

Downgrade technique of braking firmly to reduce speed about 5 mph below the safe target, then releasing and letting the engine brake hold speed until it climbs back, then repeating. It limits heat and air use.

Device that uses engine compression or exhaust restriction to slow the vehicle without using wheel brakes, reducing heat and air consumption on grades. It does not replace service brakes for stopping.

Loss of braking power caused by overheated drums and linings, common after prolonged or repeated hard braking on downgrades. The pedal may feel normal while stopping power drops, so speed must be reduced early.

Controlled stop made when air pressure is lost or brakes fail, using engine braking, downshifting, and available remaining braking while steering to a safe place. Spring brakes may set automatically as pressure falls.

Emergency braking method of pressing the pedal fully and holding it, rather than pumping, so ABS can work and full air pressure reaches the chambers. Pumping defeats antilock operation and wastes air.

Electronic system that modulates brake pressure to prevent wheel lockup during hard braking, helping the driver keep steering control. A yellow ABS lamp means the system may be disabled and should be checked.