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 secondary sys-
tem. Loss of secondary system air pressure causes the front axle brakes to become
inoperative; rear service brakes and trailer brakes will be operated by the primary
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 vehicles 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.
On tractor-trailer vehicles, if both the primary and secondary sys-
tems 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
All vehicles have a four-channel anti-lock brake system (ABS) and
cam-operated service brakes with non-asbestos brakeshoes.
All vehicles except M915A2 and M916A1 have automatically adjust-
ing slack adjusters. On all axles, brake chambers have a stroke alert indicator which
allows the operator to monitor brakeshoe wear.