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PRESSURE PROTECTION
RES.
#1
RELAY VALVE
FIGURE 2 - SR-4
™
VALVE CROSS-SECTIONAL SCHEMATIC
units. Air is prevented from fl owing from the SR-4
into reservoir #1 by check valve "C". Air simultaneously
fl ows into the PR-3
™
pressure protection valve, (Type 1
SR-4
™
valve) or the second pressure protection piston
(Type 2 SR-4
™
valve). At approximately 70 psi piston "F"
moves against the resistance of its spring, allowing air to
fl ow through check valve "G" and into reservoir #1.
PARK APPLICATION
To apply the trailer spring brakes, the trailer supply line
is exhausted by means of one of the cab controls. Air
pressure is removed from the control piston and the
internal pressure protection piston, closing the pressure
protection valve and the spring brake inlet valve.
SERVICE SYSTEM FAILURE
If air pressure is reduced in the service system, pressure in
the trailer supply line (and in the tractor) will be reduced until
the internal pressure protection valve closes, maintaining
50-60 p.s.i. in the trailer supply line and the tractor service
reservoir. This will maintain 50-60 p.s.i. on the control piston
which will keep the spring brakes released. At the same
time, the reduction in pressure in the tractor system will
sound the low pressure signal, alerting the driver. If the
service failure is in the #2 reservoir, the air which passes
the pressure protection valve will escape through the #2
reservoir and will not be able to resupply the #1 reservoir.
The #1 reservoir will retain full pressure because of the
pressure protection check valve and check valve "B". This
2
PRESSURE
PROTECTION
INLET VALVES
PISTON "F"
CHECK VALVE "G"
RES.
#2
CHECK VALVE "C"
SPRING BRAKE
INLET VALVE "E"
SPRING BRAKE
PRESSURE PROTECTION
PISTON SPRINGS
™
valve
reserve supply of air will provide at least one spring brake
release after a manual application. Service applications
may still be made until the air in the #1 reservoir is
exhausted.
If the service failure is in the #1 reservoir, the pressure
protection valve will close and allow air to supply the
#2 reservoir. This air will be available to release the
spring brake through check valve "B", although service
applications may not be made.
EMERGENCY APPLICATION WITH SERVICE
SYSTEM FAILURE
To brake the trailer after a service system failure, the
remaining 55 p.s.i. pressure in the trailer supply line may
be exhausted either manually or automatically by the trailer
supply valve or parking control valve. The spring brakes are
then exhausted through the control piston and thus applied.
Suffi cient pressure will be maintained in the trailer system
to release the spring brakes at least once.
ANTI-COMPOUNDING OPTION
Under certain conditions when the tractor protection valve
goes into the emergency mode during a service application,
the trailer supply line will be exhausted but some air may
be trapped in the service line, creating a service application
plus a full spring application. This creates a false park
situation and adds an extra load on the foundation brake
components. This may be prevented by adding the optional
anti-compounding piping as shown in Figs. 2 & 3.
SERVICE LINE
ANTI-COMPOUND
LINE
PRESSURE PROTECTION
PISTON "F"
CHECK VALVE "A"
CHECK VALVE "B"
ANTI-COMPOUNDING
CHECK VALVE
SUPPLY PORT
TRAILER SUPPLY LINE
CONTROL PISTON "D"
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