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DUAL RELAY VALVE
The dual relay valves (one for front and one for rear
brake circuits) supply the apply pressure for each disc
brake assembly. When the operator depresses the
brake valve, hydraulic pressure, proportional to the
amount of brake valve application, is applied to the
pilot pressure circuit of each dual relay valve. Regu-
lated pressure (proportional to the pilot pressure ap-
plied) is then delivered from the "B1" and "B2" ports of
each dual relay valve to each wheel.
The regulated pressures supplied to each wheel are
equal. If a malfunction occurs, causing a pressure
differential greater than 300 psi (2068 kPa) between
the right and left brake apply circuit, a differential
pressure switch (3, Figure 3-11) mounted on the valve
activates a warning horn and lamp in the cab. If a
pressure differential greater than 300 psi (2068 kPa)
occurs in the pilot circuit supplying the front and rear
dual relay valves, a differential pressure switch
mounted on the brake valve activates the warning horn
and lamp in the cab.
The dual relay valve for the front brake circuit is located
in the hydraulic components cabinet behind the cab.
The dual relay valve for the rear brake circuit is located
in the rear axle housing and requires removal of the
wheel motor cooling air duct components for access
to the valve as described below.
Before disconnecting pressure lines, replacing
components in the hydraulic circuits, or installing
test gauges, always bleed down hydraulic steering
and brake accumulators. The steering accumula-
tors can be bled down with engine shut down,
turning the key switch "Off" and waiting 90 sec-
onds. Confirm the steering pressure is released by
turning the steering wheel - No front wheel move-
ment should occur.
Open bleed down valves (6 & 7, Figure 3-1) located
on the brake manifold. This will allow both accu-
mulators for the rear brakes and both accumula-
tors for the front brakes to bleed down.
Before disabling brake circuit, be sure truck
wheels are blocked to prevent possible roll-away.
J03019 1/99
Removal
1. Securely block the wheels to prevent possible
roll-away.
2. Place selector switch in NEUTRAL, turn the rest
switch ON. Turn key switch OFF to shut down
engine and allow 90 seconds for steering system
accumulators to bleed down. Open valves (6 & 7,
Figure 3-1) to bleed down all four brake accumu-
lators. Close valves after all pressure is released.
NOTE: Follow steps 3 & 4 below for the front valve or
steps 5 through 9 for rear valve removal.
Front Brake Circuit:
3. Tag and remove all hydraulic lines from dual relay
valve. Plug lines and ports to prevent possible
contamination. Disconnect wiring harness at dif-
ferential pressure switch connector.
4. Remove two capscrews and washers securing
valve to rear wall of cabinet. Remove valve and
move to clean work area for disassembly.
Rear Brake Circuit:
5. Open rear access cover and remove wing nuts
securing duct tube to axle housing. Remove tube.
6. Open access covers inside axle housing.
7. Remove wheel motor cooling air exhaust duct
from between wheel motors.
8. Tag and remove all hydraulic lines from dual relay
valve. Plug lines and ports to prevent possible
contamination. Disconnect wiring harness at dif-
ferential pressure switch connector.
9. Remove two capscrews and washers securing
valve to mounting bracket. Remove valve and
move to clean work area for disassembly.
Installation.
1. Install dual relay valve in hydraulic components
cabinet (front brake circuit) or rear axle housing
(rear brake circuit). Install the two mounting cap-
screws and lockwashers to secure valve. Tighten
capscrews to standard torque.
2. Remove hose and fitting caps and plugs and
attach hoses to the proper valve ports. Connect
differential pressure switch connector to wire har-
ness.
3. Start engine and check for leaks and proper brake
operation. Shut down engine.
4. For the rear valve, reinstall the wheel motor cool-
ing air duct between wheel motors. Close access
covers and reinstall tube in axle access opening.
Brake Circuit Component Service
J3-15
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Troubleshooting
