Height-Sensing Brake Proportioning Valve - GMC G Service Manual

5A-10 HYDRAULIC BRAKES
Valve Warning Switch Test (Figure 8)
• Raise the vehicle.
• Support with suitable safety stands.
1. Attach a hose to a rear brake bleeder screw.
• Immerse the opposite end of the hose into a
container partially filled with clean brake fluid.
• Make sure the master cylinder reservoir is full.
2. Turn the ignition key to "ON".
• Open the bleeder screw while a helper applies
moderate pressure to the brake pedal.
• The warning lamp should light.
• Close the bleeder screw before the helper
releases the brake pedal. Release the pedal.
3. Apply the brake pedal with moderate to heavy
pressure.
• The lamp should go out.
4. Attach a hose to a front brake bleeder screw.
• Immerse the opposite end of the hose into a
container partially filled with clean brake fluid.
• Make sure the master cylinder reservoir is full.
5. Open the bleeder screw while a helper applies mod­
erate pressure to the brake pedal.
• The warning lamp should light.
• Close the bleeder screw before the helper
releases the brake pedal. Release the pedal.
7. Apply the brake pedal with moderate to heavy
pressure.
• The lamp should go out.
8. Turn the ignition key to "OFF".
• If the warning lamp does not light during steps 2
and 5 but does light when a jumper is con­
nected to ground, the warning switch portion of
the valve is malfunctioning. Do not disassemble
any portion of the valve. It must be replaced.
• Lower the vehicle.
• Check and fill the master cylinder to the proper level.
A. Fuse
B. Indicator Lamp
C. Brake Pressure Switch; Closed
With Unequal Brake Pressure
Figure 8 — Brake Pressure Warning Circuit
B
L00551
VALVE REPLACEMENT
H Remove or Disconnect (Figure 9)
• The combination valve is not repairable and must be
replaced as a complete assembly.
• Care must be taken to prevent brake fluid from con­
tacting any painted surface.
1. Hydraulic pipes.
• Plug the pipes to prevent the loss of fluid or the
entrance of dirt.
2. Warning switch harness.
3. Bolts.
4. Combination valve.
H Install or Connect (Figure 9)
1. Position valve on the bracket.
2. Bolts.
H O Tighten
• Bolts to "Specifications" at the end of this
section.
3. Warning switch harness.
4. Hydraulic pipes.
5. Bleed the brake system. Refer to "Bleeding Brake
System" in this section.
HEIGHT-SENSING BRAKE
PROPORTIONING VALVE
The height-sensing brake proportioning valve is used on
series 30 models (figures 10 and 11). This valve provides
optimum brake balance and efficiency. The vehicle braking
force is distributed to the front and rear wheels as deter­
mined by either a light or heavy payload condition.
The valve is mounted on the frame, and a linkage con­
nects the valve to a bracket mounted on the axle.
CAUTION: Adding any suspension accessories
or other equipment (such as load-leveling kits,
air shocks, suspension lift kits, additional
spring leaves, etc.), or making modifications
that change the distance between the axle and
the frame without changing the load will provide
a false reading to the brake proportioning valve.
This could result in unsatisfactory brake perfor­
mance, which could result in an accident and
personal injury.
VALVE REPLACEMENT
Remove or Disconnect (Figure 11)
• Raise the vehicle.
• Support the frame with suitable safety stands. The
axle must be allowed to hang free.
• Clean the exterior of the valve to prevent dirt from
contaminating the hydraulic system.