Class A: Air Brakes

Air compressor

The engine-powered pump that fills the air tanks with compressed air for the brake system

Air compressor governor

The device that controls when the compressor pumps air, cutting in around 100 psi and cutting out around 125 psi

Air tanks

Storage tanks holding compressed air used to apply the brakes

Supply pressure gauge

The gauge showing how much air pressure is in the air tanks

Application pressure gauge

The gauge showing how much air pressure is being used to apply the brakes

Low air pressure warning

A warning light and buzzer that activates when air pressure drops below approximately 60 psi

Safety relief valve

A valve that releases air automatically if pressure gets too high, protecting the tanks from bursting

Alcohol evaporator

A device that puts a small amount of alcohol into the air system to help prevent ice from forming in cold weather

Wet tank (supply tank)

The first air tank the compressor fills; collects moisture and oil, drained daily

Foot valve (treadle valve)

The brake pedal that controls the amount of air sent to the brake chambers when pressed

Brake chamber

The component that converts air pressure into mechanical force to apply the brakes

Slack adjuster

The linkage that converts brake chamber push-rod movement into the turning motion that applies the brake shoes

S-cam brakes

A drum brake design where an S-shaped camshaft pushes the brake shoes out against the drum

Dual air brake system

A system with two separate air systems (primary and secondary) for redundancy if one fails

Parking brake control (yellow knob)

The diamond-shaped yellow knob that applies spring brakes to hold the vehicle when parked

Spring brakes

Powerful mechanical springs that automatically apply the brakes if air pressure drops too low

Tractor protection valve

A valve that automatically shuts off air to the trailer if a serious air leak occurs, protecting the tractor's braking ability

Trailer air supply control (red octagon knob)

The red knob that controls air supply to the trailer's brakes; pushing it in supplies air, pulling it out cuts it off and applies trailer emergency brakes

Air dryer

A device that removes moisture from the air system before it reaches the tanks, reducing freezing and corrosion

Brake fade

Loss of braking power from overheated brakes, common after excessive downhill braking

Drain valve (petcock)

A manual valve at the bottom of an air tank used to drain moisture and oil

Automatic drain valve

A valve that automatically expels moisture and oil from an air tank without manual draining

Air leak rate test

A check performed with the engine off to ensure air pressure doesn't drop too fast, indicating a leak

Pressure gauge check

Confirming the compressor builds air pressure at a proper rate, typically from 85 to 100 psi within a set time