BRAKES - TM-9-2320-303-24-10044

TM 9-2320-303-24-1

THEORY OF OPERATION - CONTINUED

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BRAKES

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The dual air brake system consists of two independent air brake systems that use a single set of brake controls.

Each system has its own reservoirs, plumbing, and brake chambers. The primary system operates the service

brakes on the rear axle; the secondary system operates the service brakes on the front axle. On tractor-trailer con-

figurations, service brake signals from both systems are sent to the trailer.

Loss of air pressure in the primary system causes the rear service brakes to become inoperative; front brakes will

continue to be operated by secondary system air pressure. In addition, trailer brakes will be operated by the sec-

ondary system. Loss of secondary system air pressure causes the front axle brakes to become inoperative; rear ser-

vice brakes and trailer brakes will be operated by the primary system.

The warning light and buzzer inside the cab come on if air pressure drops below 64 psi (441 kPa) in either system.

If this happens, check the air pressure gages to determine which system has low air pressure. Although the vehi-

cle's speed can be reduced using the foot brake control pedal, either the front or rear service brakes will not be

operating, causing a longer stopping distance. Bring the vehicle to a safe stop and have the air system repaired

before continuing.

If both the primary and secondary systems become inoperative, the trailer service brakes or spring parking brakes

will automatically apply when air pressure drops to 35-45 psi (241-310 kPa). The tractor spring parking brakes will

automatically apply when air pressure drops below 45 psi (310 kPa).

The vehicle has a four-channel anti-lock brake system (ABS) and cam-operated service brakes with non-asbestos

brakeshoes.

The M915A4 has automatically adjusting slack adjusters. On all axles, brake chambers have a stroke alert indica-

tor which allows the operator to monitor brakeshoe wear.

AIR GAGES

SECONDARY PRIMARY

BRAKE

REAR AXLE SPRING

CONTROL

BRAKE CHAMBERS

FRONT AXLE

BRAKE CHAMBERS

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