Michigan CDL Air Brakes key terms

An engine-driven pump that draws in outside air and forces it into the storage tanks, building and maintaining the pressure the brake system needs to apply the brakes.

A valve on the compressor that monitors tank pressure and turns the compressor's pumping action on at a low cut-in setting and off at a higher cut-out setting to control pressure buildup.

A device mounted after the compressor that removes moisture and oil vapor from compressed air before it reaches the storage tanks, helping prevent corrosion and winter freeze-ups in the lines.

A sealed storage cylinder that holds compressed air for the brake system, including wet tanks near the compressor and dry tanks that supply the brake circuits and other air-operated devices.

The first storage tank that compressed air enters, so named because moisture tends to condense there; it usually has a drain cock and protects downstream tanks from water and oil.

A manually or automatically operated valve at the low point of a storage tank that lets the driver release accumulated water and oil so the tank does not corrode or pass moisture into the system.

A spring-loaded relief valve on the air system that opens at a set pressure to release excess air, protecting tanks and lines if the governor fails and pressure keeps climbing.

A dash instrument that shows the pressure in the brake system's tanks; the driver watches it to confirm normal operating pressure and to detect a loss of pressure while driving.

The treadle the driver pushes to apply the brakes; it meters air from the supply tanks into the brake chambers in proportion to how far the pedal is pressed.

A cylinder at each wheel where air pressure pushes a diaphragm and pushrod to apply the brake; it converts air pressure into the mechanical force that moves the slack adjuster.

The rod extending from the brake chamber that moves outward when air pressure acts on the diaphragm, transferring force to the slack adjuster to apply the foundation brake.

The lever arm that links the brake chamber pushrod to the brake camshaft, multiplying force and adjusting for wear so the brakes stay properly applied as linings wear.

A self-adjusting slack adjuster that takes up clearance as brake linings wear, reducing the need for manual adjustment but still requiring inspection for correct pushrod travel.

A foundation brake in which a rotating cam shaped like an S spreads the brake shoes against the drum; the cam is turned by the slack adjuster when the pushrod extends.

The rotating metal cylinder attached to the wheel hub that the brake shoes press against; friction between the shoes and drum slows the wheel and converts motion into heat.

The curved metal backing that carries the friction lining and presses against the inside of the brake drum when the S-cam spreads the shoes apart to slow the wheel.

The friction material bonded to the brake shoe that contacts the drum; it wears over time and must be thick enough to stop the vehicle without metal-to-metal contact.

A valve that speeds up brake application and release by sending air from a nearby tank directly to the brake chambers, rather than routing all the air through the foot valve.

A valve mounted near the brake chambers that exhausts air quickly when the pedal is released, shortening brake release time and helping the brakes respond promptly to driver input.

A parking and emergency brake that uses a strong spring to apply the brake when air pressure is removed; air pressure must be present to release it and let the vehicle roll.

The dash control, usually a yellow diamond-shaped knob, that releases air from the spring brakes to apply them for parking and can be used to make an emergency stop.

A buzzer and light that activate when brake system pressure drops below a set level, warning the driver to stop before spring brakes apply automatically and control is lost.

A device that adds alcohol vapor to the air system to lower the freezing point of moisture, helping prevent ice from blocking valves and lines in cold weather.

The amount of pressure the system loses per minute with the engine off and brakes applied; excessive loss signals a leak that must be repaired before driving.

A check performed with the engine off and system charged, watching the gauge for pressure drop with and without the brakes applied to confirm the system holds air.

A parked test in which the driver charges the system, shuts off the engine, waits, and watches the gauge to see whether pressure falls faster than the allowed rate.

The time the compressor takes to raise tank pressure from a low reading to the governor cut-out; a slow build-up indicates a compressor or leak problem.

The distance the brake chamber pushrod extends when the brake is applied; too much travel means the brakes are out of adjustment and may not apply with full force.

Setting the slack adjuster so pushrod travel stays within the manufacturer's limit, ensuring the brake chamber has enough stroke left to apply the brakes fully.

Loss of stopping power that occurs when overheated brake drums and linings lose friction, often after repeated or prolonged braking on long downgrades.

A downhill technique of applying the brakes firmly to reduce speed, then releasing them to cool, repeated as needed instead of riding the brakes continuously.

A device such as an engine brake or exhaust brake that slows the vehicle using engine compression, reducing reliance on the service brakes and helping prevent overheating on downgrades.

A system that applies the spring brakes when air pressure is lost or when the driver uses the parking brake valve, allowing a controlled stop if service brakes fail.

An optional dash gauge that shows how much air pressure the driver is applying to the brakes, helping the driver judge braking force and avoid over-braking.

The lower pressure at which the governor restarts the compressor to rebuild tank pressure after it has fallen from the higher cut-out setting.

The upper pressure at which the governor stops the compressor from pumping, preventing overpressurization of the tanks and lines during normal operation.

A valve that closes the air supply to the trailer when pressure drops or the trailer is disconnected, keeping the tractor's air so its brakes can still be used.

The walk-around and in-cab checks before driving that verify air pressure builds, leaks are absent, warning devices work, and brakes apply and release properly.