TM 5-4930-218-14
braking occurs. When pressure is released entirely,
though one section of the system fails. This dual
the brakes release.
system incorporates one relay valve, two air reser-
c. Each brake master cylinder controls one wheel
voirs, two air brake chambers, two master cylinders,
cylinder at each wheel. This helps to insure that even
two brake cylinders in each wheel, and a dual system
braking will occur even though one of the dual brake
of brake lines to interconnect each master cylinder
systems fails.
with one wheel cylinder at each wheel.
d. In the event that you have a breakaway or other
b. The brake system is normally operated by the
trouble which disconnects the emergency air hose
brake system of the towing vehicle to which you must
between the towing vehicle and the trailer, the relay
connect the service and emergency hoses from the
valve will cause both portions of the dual system to
trailer brake system. Pressure in the dual air reser-
apply the brakes to bring the trailer to a stop.
voirs is maintained through the emergency hose,
When the brake system of the towing vehicle is ap-
plied, air pressure is applied to the relay valve
through the service hose. This operates the relay
a. Removal. Remove the air lines and fittings that
valve, causing it to apply pressure to both air brake
interconnect the lubricating and servicing unit brake
chambers. When the push rods of the air brake cham-
system components and the towing vehicle as follows:
ber extend, they operate the brake master cylinders
(1) Open the drain cock (13, fig. 4-21) at the bot-
to operate the wheel cylinders. The pressure applied
tom of the lower air reservoir (17) to drain all air
to the relay valve through the service hose controls
from the brake air system before disconnecting hoses.
the braking force. When low pressure is applied, light
(2) Remove the air couplings (1, fig. 4-20).
braking occurs. When high pressure is applied, heavy
