1-15. AIR SYSTEM - Continued
b. Driving Accessory Air System. Air supply for operation of the air horns is provided from the supply
reservoir via a manifold.
1-16. BRAKE SYSTEM. Air pressure to operate the service and parking brakes is supplied from the rear
service, rear service EXT. 1, rear service EXT. 2, front service and isolated emergency spring brake release
reservoirs. During normal operations, air in the front system is used to apply the front brakes and release the
parking brake; the air in the rear system is used to apply the rear brakes.
a. Parking Brakes.
(1) To release the parking brakes, the parking brake control is pushed IN. Air from the rear service
reservoir flows from the control valve via the spring brake valve, double check valve and quick release
valve, and service relay to the rear brake air chambers. To engage the brakes, the parking brake
control is pulled OUT to vent the brake air chamber to the atmosphere. As the air pressure is relieved,
spring pressure from the spring chambers engages the brakes.
(2) If the rear service air supply is depleted the brakes can be released by pressure from the isolated
emergency spring brake release reservoir. By pushing and holding the parking brake control IN, the
front service is isolated and the emergency reservoir is opened to pressurize the system.
b. Rear Brake System.
(1) When the brake pedal is depressed, pilot pressure is supplied from the rear service reservoir via the
service brake valve and pressure protection valves to the relay valve. The pilot pressure opens the
relay valve and allows brake application air from the rear service reservoir to enter and pressurize the
spring brake chambers. The combined force of air and spring pressure suppresses the parking brake
release pressure in the opposing air chambers, and engages the brakes.
(2) To prevent excessive brake force, the continuously balanced pilot and spring chamber air pressures are
regulated to a maximum 45 psi (310 kPa). Below this limit the pressures are infinitely controlled.
(3) When the brake pedal is released, the pilot pressure is exhausted. The relay valve shuts off the brake
application air and vents the spring chambers to atmosphere. The opposing air chambers remain
pressurized and thus disengage the brakes.
(4) To prevent jamming of the brakes if the service brakes are applied while the parking brakes are
engaged, counteracting pressure is provided to the air chambers. This air pressure is provided via the
double check valve and quick release valve simultaneously with the application of air pressure to the.
(5) If the rear service system becomes depleted of air, the front service is indirectly used to apply the rear
brakes. The shift from rear service to front service air is automatic and carried out in the spring brake
valve when the brakes are applied.
(6) Normally, primary air pressure, supplied via the relay valve, keeps the secondary (control) air supply
port in the spring brake valve closed. This action keeps the valve function neutral.
(7) With no primary air available, secondary air pressure causes the spring brake to partially close and
reduce the air pressure that keeps the parking brakes disengaged. When this occurs, the quick release
valve automatically opens and momentarily vents the air chambers to the atmosphere. The reduction
in pressure in the air chambers is proportional to the secondary air pressure applied to the spring brake