Air is pumped by the compressor to the reservoir. When air pressure reaches 120 to 145 psi (828 to 1,000 kPa), the governor places the compressor into its unloaded stage. At this stage the air system is fully charged (Fig. 14).
When the brakes are applied, air is delivered through the foot valve to the service-brake chambers (Fig. 15). Air pushes against each service-brake diaphragm causing the push rod to move the slack adjuster (see page 55). The slack adjuster rotates the brake cam, which forces the shoes against the brake drum.
When you release the foot valve, air in the brake chambers is exhausted through the foot valve, which releases the brakes.
When reservoir air pressure drops, the governor puts the compressor back into the pumping stage to keep adequate air pressure available for future brake applications.