MS CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, usually governed to stop pumping around 120-125 psi and restart near 100 psi. It supplies the reservoirs through the air dryer.

Valve that monitors system pressure and tells the compressor when to pump and when to stop, controlling the cut-in and cut-out pressure range. A cut-out near 120-125 psi and cut-in near 100 psi is typical.

Canister between the compressor and the first reservoir that removes water vapor and oil from compressed air. Its desiccant and purge cycle protect the system from internal corrosion and freeze-ups in cold weather.

Tank that stores compressed air for the system, also called the wet tank because moisture tends to collect there. It feeds the primary and secondary circuits and is protected by a drain valve and safety valve.

One of the two separate air circuits in a dual system, usually supplying the rear service brakes and sometimes trailer brakes. Its reservoir is charged through a one-way check valve so a failure elsewhere cannot drain it.

The second independent air circuit in a dual system, typically supplying the front service brakes. Because it has its own reservoir and check valve, a leak in the primary circuit still leaves it able to stop the vehicle.

Design that splits the service brakes into two isolated air circuits fed from separate reservoirs, so a single leak or failure cannot disable all service braking. Each circuit can still apply some brakes on its own.

Device that allows air to flow in only one direction, used to keep each reservoir charged while preventing air from escaping backward through a failed circuit. It is central to the isolation that makes a dual system safe.

Pressure-relief valve on the supply reservoir that opens if the governor fails and pressure climbs too high, protecting tanks and lines from rupture. It is normally set well above the compressor cut-out pressure.

Dashboard instrument showing pressure in the brake circuits, often with separate needles for primary and secondary. The driver watches it to confirm the compressor is building pressure and that no circuit is leaking down.

Audible and visual alert, usually a buzzer plus a red light, that activates when system pressure drops to about 60 psi or below. It warns the driver that braking ability is fading and a stop must be made.

Parking and emergency brakes held released by air pressure and applied by heavy springs when pressure is lost. They apply automatically at low pressure, which is why a severe leak brings the vehicle to a stop.

Dash valve, usually a yellow diamond-shaped knob, that releases air to set the spring brakes and supplies air to release them. Pushing it in charges the spring brake chambers; pulling it out applies them.

Valve that controls how fast air escapes from the spring brake chambers, so the emergency springs apply gradually instead of slamming on. It prevents a sudden lockup and loss of control during an automatic application.

Treadle valve operated by the brake pedal that meters air from the reservoirs to the brake chambers according to how hard the pedal is pressed. Releasing the pedal exhausts the air and releases the brakes.

Valve mounted near the rear brakes that uses a small control signal from the foot valve to send a large volume of air to the brake chambers. It speeds up application and release on long vehicles.

Valve placed close to the brake chambers that dumps exhaust air locally when the pedal is released, instead of sending it all the way back through the foot valve. This shortens release time and reduces brake drag.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the foundation brake. In spring brake chambers a second section holds the emergency spring that applies the brake when air is lost.

Link between the brake chamber pushrod and the slack adjuster arm that converts pushrod stroke into rotation of the brake camshaft. Its adjustment determines how far the pushrod must travel to apply the brake.

Distance the brake chamber pushrod travels when the brake is fully applied, measured to judge adjustment. Too long a stroke means the brake is out of adjustment and stopping power is reduced.

Setting of the slack adjusters so pushrod stroke stays within the limit marked on the chamber or in the manual. Automatic slack adjusters should keep this correct, but drivers still must check and report long strokes.

Walk-around check of chambers, hoses, lines, slack adjusters, drums, linings, and drain valves before driving. It looks for cracks, leaks, chafing, missing parts, and long pushrod strokes that would make the vehicle unsafe.

Cock at the bottom of each air tank used to blow off accumulated water and oil, usually daily. Draining until only clean air escapes keeps moisture from corroding tanks and freezing in lines.

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

Check made with the engine off, system fully charged, and brakes released, watching the gauges for pressure drop over one minute. A steady loss beyond the allowed limit means a leak must be found and fixed.

Check made with the system charged and the brake pedal held fully down for one minute, watching for pressure drop. It finds leaks in the service brake chambers, lines, and valves that a released-brake test would miss.

Amount of pressure lost per minute during a leak test, compared against the limit in the CDL manual for the vehicle. A leak rate above the limit, or any single-circuit drop, means the vehicle should be placed out of service.

Check that with the engine running at a fast idle, pressure rises from about 85 to 100 psi within the time stated in the manual. Slow build-up points to a weak compressor, slipping belt, or serious leak.

Procedure of pumping the brakes with the engine off to confirm the warning buzzer and light come on by about 60 psi, and that the spring brakes apply. It proves the safety features work before a trip.

Valve that closes the tractor's air lines to the trailer if the trailer breaks away or a line separates, keeping tractor air from escaping. It also lets the trailer spring brakes apply automatically in that situation.

Two hoses, service (blue) and emergency or supply (red), that carry control and supply air between tractor and trailer. They must be coupled correctly, supported, and free of chafing or leaks.

Coupling fittings that join the tractor and trailer air hoses, with a rubber seal in each half. They must be locked together and checked for leaks, and the lines must not cross or drag.

Loss of stopping power when overheated drums and linings cannot create enough friction, common after repeated hard braking on long downgrades. Using a lower gear and controlled braking helps prevent it.

Downgrade technique of braking firmly to reduce speed about 5 mph, then releasing the pedal and letting speed build again before repeating. It gives brakes time to cool and avoids continuous light drag that overheats them.

Emergency technique of pressing the brake pedal fully, holding it, and releasing only when the wheels lock or the vehicle is nearly stopped. It is used when stopping power is fading and a normal stop is not possible.

Device such as an exhaust or compression brake that slows the vehicle using engine resistance, saving service brakes for when they are needed. It does not replace the brakes and may be restricted in some areas.

Controlled stop made when brakes fail or pressure is lost, using downshifting, retarder, spring brake application, and safe off-road escape routes. The goal is to slow gradually and warn others rather than stop instantly.

Test stop made at low speed soon after starting, using the brakes to confirm the vehicle pulls up straight and stops normally. It reveals problems like a weak chamber or contaminated lining before they matter at highway speed.