Air Brake Manual

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Basic system components

The five main components of an “elementary” air brake system and their purposes are:

  1. Compressor: to build up and maintain air pressure
  2. Reservoirs: to store the compressed air
  3. Foot valve: to draw compressed air from reservoirs when it is needed for braking
  4. Brake chambers: to transfer the force of compressed air to mechanical linkages
  5. Brake shoes and drums or brake rotors and pads: to create the friction needed to stop the vehicle


The function of the air compressor (Fig. 8) is to build up and maintain air pressure required to operate air brakes and air-powered accessories.

Air compressors are either gear driven directly from the engine or belt driven. Although most compressors use the truck’s lubrication and cooling systems, some are self-lubricated and some are air cooled. Self-lubricated compressors must have their oil checked and changed at regular intervals.

The compressor’s intake system draws air from either its own air filter or from the engine’s intake system.

Compressors that have their own filtration system must be serviced on a regular basis.

All compressors run continuously while the engine is running, but air compression is controlled and limited by a governor which “loads” or “unloads” the compressor. In the loaded stage, air is pumped into reservoirs. In the unloaded stage (with two cylinder compressors), the compressor pumps air back and forth between the two cylinders without supplying the reservoirs.

Air compressor
Figure 8. Air compressor


The governor must take the compressor out of its pumping stage (unload/cut-out) when system air pressure reaches 120 to 145 psi (828 to 1,000 kPa), and also put it back into the pumping stage at a minimum of 100 psi (690 kPa).


Reservoirs are pressure-rated tanks, which hold a supply of compressed air until required for braking or operating auxiliary air systems. They must store a sufficient volume of air to allow several brake applications if the engine stops or the compressor fails.

The maximum air pressure available for brake applications depends on how much air is in the reservoir. A driver is not able to make a higher pressure brake application than there is air pressure in the reservoir.

Each reservoir is equipped with a drain valve called a draincock (Fig. 9). Fully opening the draincock allows reservoirs to be drained of moisture and other contaminants that build up in the system. All reservoirs must be completely drained once a day when in use.

Typical reservoir drain valves
Figure 9. Typical reservoir drain valves

Foot valve (application or treadle valve)

This foot-operated valve (Fig. 10) applies air to operate the brakes. The amount of air delivered to the brakes is regulated by the driver according to the distance the treadle or brake pedal is depressed. Releasing it exhausts air in the service brakes through its exhaust port.

These valves are made in overhead styles with a foot pedal hanging down, or a floor-mounted version with a foot treadle.

Dual-circuit foot valve
Figure 10. Dual-circuit foot valve

Service-brake chambers (brake pots)

Service-brake chambers (Fig. 11) convert compressed air pressure energy into mechanical force and movement, which apply the vehicle’s brakes.

When you press down on the foot valve, air pressure enters the pressure side of the brake chamber through the inlet port and forces against the diaphragm, which moves the push rod assembly forward. When air pressure is released from the service-brake chamber, the return spring returns the diaphragm and push rod to their released positions.

Clamp-ring type service brake chamber
Figure 11. Clamp-ring type service brake chamber

Brake shoes and drums

Figure 12 illustrates the common S-cam brake assembly used on truck and trailer axles. Front brake assemblies have the brake chamber and slack adjuster mounted on the backing plate because the steering action of the front axle would otherwise interfere.

The diagram shows the brakes in the applied position. The S-cam is rotated so the high points have acted against the cam rollers and forced the brake shoes against the drum.

When the brakes are released, the brake cam shaft returns the brake cam to the normal position. The cam rollers roll down into the crook of the S-cam as the brake shoe return spring pulls the shoes away from the drum.

Brake lining material is attached to the face of the shoes. Lining material is selected according to the type of service the brakes are subjected to. Linings must give consistent braking output with minimum fade at high temperatures.

Brake shoes generate heat through friction with the brake drum surface. Drum thickness determines the amount of heat that can be absorbed and dissipated to the atmosphere. Thin or distorted drums, weak return springs, improper linings, poor adjustment, or grease or dirt on the lining, will all result in erratic, unpredictable and potentially dangerous brake performance.

S-cam brake assembly
Figure 12. S-cam brake assembly


Rev: 2018