Air pressure pushes out a rod that moves the slack adjuster and twists the S-cam, and the S-cam spreads the brake shoes apart. The shoes and linings then rub on the inner surface of the brake drum, creating the friction that slows the wheels; they do not press against the S-cam or the slack adjuster.
An air brake system has three parts: service brakes, parking brakes, and emergency brakes. ABS is an extra electronic control that helps keep the wheels from locking up, not one of those three braking systems.
The evaporator adds alcohol to the air supply so that moisture is less likely to freeze in the valves and lines when it is cold; ice inside the system can block the air and keep the brakes from working. It does nothing for tire grip or brake cooling.
The compressor is linked to the engine through gears or a V-belt, and the engine's power lets it pump air into the storage tanks. Some compressors are cooled by the engine's cooling system, but the radiator does not drive the compressor, and neither does the drive shaft.
A one-way check valve lets air pass from the compressor into the tanks but not back out, so a compressor leak cannot drain the stored air. The tractor protection valve protects the tractor's air if the trailer breaks away or leaks badly, and the safety relief valve vents air when pressure gets too high.
These tanks, also known as reservoirs, keep a supply of compressed air ready for braking and hold enough for several brake applications if the compressor stops working. The governor is what tells the compressor when to stop pumping, and the compressor is the part the engine drives.
The safety valve is fitted to the first tank the compressor fills and is normally set to vent at 150 psi, well above the governor's cut-out point of about 125 psi. It lets air escape only if a faulty compressor or governor allows pressure to climb too high.
Air brake systems can use S-cam drum, wedge or disc brakes at the wheels, but the S-cam drum design is by far the one seen most often on air-braked vehicles.
This valve guards the tanks against excess pressure and is usually set to open at about 150 psi. If it releases air, pressure is climbing higher than it should, which points to a fault that a mechanic needs to fix.
The storage tanks keep a reserve of compressed air for the brakes, large enough that you can still brake several times after a compressor failure and bring the vehicle to a safe stop.
The governor turns the compressor's pumping on and off: it stops the pumping once tank pressure reaches the cut-out level (around 125 psi) and lets it resume when pressure falls to the cut-in level (around 100 psi). How hard the brakes apply is controlled by the brake pedal, not by the governor.
Air brakes are really three systems in one: the service brake for everyday slowing and stopping, the parking brake that keeps a parked vehicle from rolling, and the emergency brake, which draws on parts of the other two to stop the vehicle if the brake system fails.
Air brakes work with compressed air and provide the strong stopping power that big trucks and buses need. They are standard equipment on many heavy vehicles and are used on every kind of road surface, not only wet ones.
The safety relief valve protects the first air tank by opening only when pressure climbs too high, usually around 150 psi. If it is venting air, part of the system is not working right, and a mechanic should repair it.
The evaporator puts a small amount of alcohol into the air system, which lowers the chance that moisture will freeze in valves and other parts during cold weather; ice there can keep the brakes from working. Air tanks still need regular draining unless they have automatic drain valves.
Older trucks with this control could cut front brake pressure in half for slick roads, but doing so only reduces stopping power. Front-wheel braking works well in all conditions, even on ice, so the control should always stay in the normal position.
Slack adjusters take up the play in the brake linkage so the shoes reach the drum after only a short stroke. During inspection, a slack adjuster that moves more than about one inch where the push rod attaches when pulled by hand is out of adjustment and needs attention.
The brake pedal, also called the foot valve or treadle valve, controls the air sent to the brakes: pushing it harder applies more air pressure, and letting up releases air and eases the brakes off. It has no effect on how fast the compressor runs, and you should not rest your foot on it during normal driving.
Every question in this set has its own page with the correct answer and an explanation.
