1-17. BRAKE SYSTEM. Air pressure to operate the service and emergency brakes is supplied from the primary,
secondary, and emergency reservoirs. During normal operation, air in the primary system is used to apply the rear
brakes and air in the secondary system is used to apply the front brakes and release the parking brakes.
a. Parking Brakes.
(1) To release the parking brakes, the control (A), (see fig. 1-30) is pushed in Air from the secondary
reservoir flows from the control valve via the spring brake valve, double check valve and quick release valve to the rear
brake air chambers. To engage the brakes, the control (A) is pulled OUT to depressurize the parking brake air lines.
When these lines lose pressure, the quick release valve opens and vents the brake air chambers to the atmosphere. As
the air pressure is relieved, spring pressure from the spring chambers engages the brakes.
(2) If the secondary air supply is depleted, the brakes can be released by pressure from the emergency
reservoir. By pushing and holding the control (B) IN, the secondary reservoir is isolated and the emergency reservoir is
opened to pressurize the system.
Figure 1-30. Parking Brake System