BRAKE SYSTEM -706880025

TM 5-4210-233-14&P-1

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.

spring chambers.

(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

valve.

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