Ohio CDL Air Brakes Test 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.

Valve on or near the compressor that senses system pressure and signals the compressor to stop or resume pumping at its cut-out and cut-in settings.

The low and high pressure points where the governor restarts and stops compressor pumping; typical settings are about 100 psi cut-in and 120-125 psi cut-out.

Storage tank that holds compressed air for braking; vehicles have at least one supply tank per circuit plus a wet tank that collects moisture and oil.

First tank that receives air from the compressor, where moisture and oil condense before air passes to the dry storage tanks for the brake circuits.

Cartridge or desiccant unit that removes water vapor and oil from compressed air before it reaches the reservoirs, helping prevent freeze-ups and corrosion.

Manually or automatically operated valve at the bottom of a reservoir used to blow off accumulated water and oil during the pre-trip inspection.

Pressure-relief valve on the air system, usually set near 150 psi, that opens to release air if the governor fails and pressure climbs too high.

Pedal-operated valve that meters compressed air from the reservoirs to the brake chambers in proportion to how far the driver pushes the treadle.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the service brakes; spring brakes add a second, mechanically released section.

Link between the brake chamber pushrod and the brake shoe that converts pushrod stroke into shoe movement and provides a way to adjust brake clearance.

Distance the brake chamber pushrod travels when the brakes are fully applied; too much stroke means adjustment is needed and braking power is reduced.

Maximum legal pushrod travel for a given chamber type and size; exceeding it places the vehicle out of service because brakes may not apply fully.

Valve mounted near the rear brakes that uses a small control signal from the foot valve to quickly deliver a large volume of air to the chambers.

Valve that exhausts air from a brake chamber close to the wheel, speeding brake release and reducing lag compared with exhausting all the way back to the treadle.

System with two separate air circuits, typically a primary for rear brakes and a secondary for front brakes, so a failure in one still leaves braking ability.

Air circuit that typically supplies the rear service brakes; it is charged first and protected by its own reservoir and check valve.

Air circuit that typically supplies the front service brakes; it is isolated from the primary so one leak cannot drain both circuits.

One-way valve that lets air flow into a reservoir but prevents it from flowing back, keeping the two brake circuits isolated from each other.

Audible and visual alert that must activate before pressure drops below 60 psi, warning the driver that braking and the parking spring brakes are at risk.

Dash instrument showing pressure in each brake circuit; the driver watches it to confirm buildup, normal operating range, and loss of pressure.

Check made with the engine off, brakes released, and system fully charged; pressure should not drop more than 3 psi in one minute for a single vehicle.

Check made with the engine off and the service brakes held fully applied; pressure should not drop more than 4 psi in one minute for a single vehicle.

Measured pressure drop per minute used to judge leaks; allowable limits are stricter for single vehicles than for tractor-trailer combinations.

Valve that closes automatically if trailer air pressure is lost or the emergency line separates, sealing the tractor's air supply and applying trailer spring brakes.

Dash valve, often red, that charges and releases the trailer brakes and can be used to test the tractor protection system by shutting off trailer air.

Air line from tractor to trailer that carries the control signal from the foot valve to apply the trailer service brakes.

Air line that supplies the trailer's reservoirs and signals a breakaway; if it loses pressure, the trailer spring brakes apply automatically.

Brake chamber with a powerful stored spring that applies the brakes when air pressure is released, used for parking and emergency braking.

Dash valve, usually yellow, that releases air from the spring brake chambers to apply the parking brakes and admits air to release them.

Automatic application of the spring brakes when air pressure falls to roughly 20-45 psi, bringing the vehicle to a controlled stop.

Valve that prevents service brake air and spring brake force from combining on the same chamber, which could overstress and damage brake parts.

Delay between pressing the brake pedal and the brakes actually applying, caused by the time air takes to travel through lines; about half a second at highway speed.

Emergency technique of pressing the brake pedal fully, holding it, and releasing only when wheels lock, then repeating; used without ABS on slippery roads.

Braking as hard as possible without locking the wheels, keeping the vehicle straight and retaining steering control; the preferred technique for ABS-equipped vehicles.

Electronic system that senses wheel lockup and rapidly adjusts brake pressure to prevent skids, helping the driver keep steering control during hard braking.

Dash light that should come on at start-up and go out above about 4 mph; if it stays on, ABS is disabled but normal air brakes still work.

Condition where service brake force and spring brake force act on the same chamber at once, risking damage; prevented by the anti-compounding valve.

Repeatedly applying and releasing the brakes, which wastes air and can leave too little pressure for a stop; not a recommended downhill technique.