1-7. Location and Description of Major Components (cont'd)
2. Relay valve
Two relay valves. One is located at the forward side of aft
air reservoir. The other relay valve is located at forward
side of forward air reservoir.
Directly controls service brakes by controlling flow of air to
and from air reservoir.
Relays are connected to emergency and service air lines, air
reservoirs and brake air chambers. Automatically apply
brakes if semitrailer breaks away from towing vehicle.
Brakes also automatically apply if there is a serious leak in
the emergency air line.
3. Ratio relay valve
Air from the emergency air hose flows into the ratio relay
valve. From there it flows to the relay valves and the air
By directing the flow of emergency air, the ratio relay valve
directly controls parking brakes.
4. Air chambers
Four air chambers (1) are located on the axle at each
of the four wheel assemblies.
Yoke assembly (2) on each air chamber acts with
attached slack adjuster to apply brakes.