PREVOST X3-45 Multiplex, Maintenance Manual

Page: 573 / 1422

7
ANTI COMPOUNDING (FIGURE 11)
The SR-7 provides anti-compounding of the service and spring
brake forces. When the park brakes are actuated (by pulling
the dash valve button out), a service brake application will
cause the SR-7 to deliver air pressure to the spring brake
chambers. Thus the vehicle is held stationary using a service
brake application. When the service brake application is
released, the delivery pressure is exhausted from the spring
brake chambers and the vehicle remains parked using the
spring brake actuators.
PREVENTIVE MAINTENANCE
Important: Review the warranty policy before performing
any intrusive maintenance procedures. An extended warranty
may be voided if intrusive maintenance is performed during
this period.
Because no two vehicles operate under identical conditions,
maintenance intervals will vary. Experience is a valuable
guide in determining the best maintenance interval for a
vehicle.
OPERATING TEST
Block vehicle and hold by means other than vehicle brakes.
Charge air brake system to governor cut-out pressure.
1. Place parking control valve in “park” position. Observe
that spring brake actuators apply promptly. Remove
one line from delivery port of the SR-7 valve and install
test gauge known to be accurate. Place parking control
valve in “release” position. Observe that spring brake
actuators release fully.
2. With parking control valve in “release” position, note
gauge pressure reading. (Correct spring brake actuator
hold-off pressure is 107 psi nominally.)
3. Place parking control valve in “park” position - gauge
reading should drop to zero promptly. A lag (more than
3 seconds) in drop of pressure would indicate faulty
operation.
4. With the parking control valve in the “park” position,
gradually apply foot brake valve and note a pressure
reading increase on the gauge installed in the SR-7
delivery port.
5. Place parking control valve in “release” position.
6. Drain the reservoir, which supplies the rear service brake
circuit, apply the foot brake valve several times and note
that pressure reading on gauge decreases each time
foot brake valve is applied (spring brake modulation).
After the foot brake valve has been applied several times,
pressure on gauge will drop to the point where release
of the spring brake actuators will no longer occur.
LEAKAGE TEST
Place the park control valve in the “release” position; using
a soap solution, coat all ports including the exhaust port. A
1 inch bubble in three seconds is permitted.
If the valve does not function as described, or if leakage is
excessive, it is recommended that it be replaced with a new
or remanufactured unit available from a Bendix parts outlet.
DO NOT ATTEMPT TO DISASSEMBLE THE SR-7. THE
VALVE CONTAINS HIGH SPRING FORCES THAT COULD
RESULT IN PERSONAL INJURY IF DISASSEMBLY IS
ATTEMPTED!
FIGURE 11 - ANTI-COMPOUNDING
NO CONTROL LINE
PRESSURE
SECONDARY RESERVOIR
CONSTANT PRESSURE
BALANCE PORT -
PRIMARY CIRCUIT PRESSURE
CONTROL PORT -
SECONDARY CIRCUIT PRESSURE
INLET / EXHAUST VALVE OPEN
DELIVERY TO SPRING BRAKES
à
à
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à à
à
à
à
à
à
à
à
à
à
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à