Bendix Commercial Vehicle Systems SR-7 SPRING BRK MOD VLV 1/11 - Manual (Page 7)

7
ANTI COMPOUNDING (FIGURE 11)
The Bendix ® SR-7 ® valve 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
® valve 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 Bendix Warranty Policy before
performing any intrusive maintenance procedures. A
warranty may be voided if intrusive maintenance is
performed during the warranty period.
No two vehicles operate under identical conditions; as
a result, maintenance intervals may vary. Experience is
a valuable guide in determining the best maintenance
interval for air brake system components. At a minimum,
the SR-7
® valve should be inspected every 6 months or
1500 operating hours, whichever comes fi rst, for proper
operation. Should the SR-7
® valve not meet the elements
of the operational tests noted in this document, further
investigation and service of the valve may be required.
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
®
valve 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.
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