a. General. This paragraph discusses the normal and emergency operation of the two brake systems found
on the trailers. The brake system used will depend on the towing vehicle.
(1) The XM979 and XM1061 can use either the air/hydraulic brake system or the inertia brake system.
(2) The M1061A1 and XM1073 use an air/hydraulic brake system only.
(3) The M1061A1 may also be supplied with the inertia brake system.
b. Air/Hydraulic Brake System (All Models).
(1) Normal Operation.
(a) When the driver of the towing vehicle applies the brakes, air pressure from the towing vehicle is
sent to the air coupling (4) and into the air hose (5) of the trailer. Air pressure in the trailer air
hoses then operates a diaphragm located in the airbrake chambers (9) next to the master
(b) The diaphragm in the airbrake chambers (9) converts air pressure to hydraulic pressure when it
pushes a piston in the master cylinders (8) and sends brake fluid through the hydraulic tubes
(7) to the wheel cylinders (1 3).
(c) The wheel cylinders (13) change the hydraulic pressure into mechanical action. When the
wheel cylinders are pressurized, they push the brakeshoes (12) against the brakedrum (14)
and stop the trailer.
(d) When the towing vehicle brakes are released, pressure is released and the trailer brakes are
(2) Emergency Operation.
(a) Emergency braking for the air/hydraulic brake system is activated when the emergency air
hose (16) is either disconnected or severed. The air pressure in the air reservoir (10) is released
into the relay valve (15) when there is a drop in pressure due to the severed or disconnected
emergency air hose. The relay valve then directs the air to the airbrake chambers (9), braking
the trailer as in normal operation.
(b) The emergency brakes can be released quickly by pulling the air tank drain handle (11). This
releases all air pressure in the air reservoir (10), thereby opening the relay valve (15).
c. Inertia Brake System (XM979 and XM1061).
(1) Normal Operation.
(a) As the driver of the towing vehicle applies the brakes, slowing down the vehicle, the actuator
(2) compresses when the trailer starts to overtake the towing vehicle. When the actuator
compresses, it forces brake fluid from the actuator's master cylinder (3) into the hydraulic
hoses and tubes (6 and 7), increasing the hydraulic pressure to the wheel cylinders (13).
(b) The wheel cylinders (13) change the hydraulic pressure to mechanical motion. When the wheel
cylinders are pressurized they push the brakeshoes (12) against the brakedrum(14) and stop
(c) The actuator (2) has a spring that acts like a shock absorber to eliminate jerky towing Iunette (1)
movement and unnecessary braking of the trailer.