Bendix SR-2 TRAILER SPRING BRAKE VALVE User Manual
Bendix® SR-2™ Trailer Spring Brake Valve
SD-03-4510
FIGURE 1
EXTERIOR
CHECK VALVE (10)
CHECK VALVES (10)
(SHOWN OUT OF
POSITION)
RES.
FTG.
(15)
O-RING (7)
INLET VALVE
SPRING (6)
SCREW (16)
VENT
INLET/EXHAUST
VALVE (5)
SECTIONAL
RETAINER (17)
SPRING (9)
PISTON (8)
O-RING (2)
SPRING (3)
BODY
(20) WASHER
(19) SCREW
COVER (12)
CAP SCREW (13)
O-RINGS (1)
CONTROL
PISTON (14)
SEAL RING (4)
EXHAUST PORT
RETAINER
DIAPHRAGM (11)
PARTIAL VIEW OF EXHAUST
FOR PC. NO. 287376
DESCRIPTION
In a trailer air brake system, the SR-2™ spring brake
valve controls the functioning of the spring brakes during
parking and emergency applications. It is responsible for
automatically applying the spring brakes and preventing
trailer air pressure loss in the event of breakaway or supply
line failure. The SR-2™ valve also prevents the automatic
application of the trailer spring brakes after the loss of trailer
service reservoir pressure while allowing failure indication
to occur in the tractor.
The SR-2™ valve is provided with one 3/4” male pipe thread
connection for mounting the valve to the trailer brake
reservoir. Other porting includes:
One - 1/4” pipe thread supply port
Four - 3/8” pipe thread delivery ports
Two - 1/4” pipe thread service reservoir ports
One - 3/8” pipe thread exhaust port
OPERATION
CHARGING (Figure 2)
Air from the trailer supply line enters the SR-2™ valve
trailer supply port and moves the control piston into contact
with the control inlet and exhaust valve, sealing off the
exhaust passage through the control piston, and opening
the inlet. Air entering the supply port is also conducted
to the underside of the pressure protection piston. When
air pressure builds to approximately 55 psi beneath the
pressure protection piston, the piston moves against the
force of the piston spring and the pressure protection inlet
valve opens and remains open. Air fl owing past the open
pressure protection valve opens the service reservoir check
valve. Air passing through the service reservoir check
1
valve fl ows out the service reservoir port and then opens
the spring brake reservoir check valve, allowing air to fi ll
the spring brake reservoir. As air is fi lling the spring brake
reservoir, it also fl ows by the open control inlet and out
the delivery ports of the SR-2™ valve into the spring brake
emergency ports releasing the spring brakes.
SERVICE
RESERVOIR
CHECK VALVE
SPRING BRAKE
RESERVOIR
CHECK VALVE
PRESS.
PROTECTION
VALVE
PRESS.
PROTECTION
INLET VALVE
CONTROL
SERVICE
RESERVOIR
PISTON
PARK APPLICATION (Fig. 3)
To park the trailer, either the trailer supply valve or the
parking control valve, which are located in the cab, are
actuated and the trailer supply line is exhausted. When
the trailer supply line is exhausted, air pressure is removed
SERVICE
RESERVOIR
CHECK VALVE
SPRING BRAKE
RESERVOIR
CHECK VALVE
SPRING
BRAKE
RESERVOIR
CONTROL INLET &
EXHAUST VALVE
SPRING BRAKE
PRESS.
PROTECTION
VALVE
PRESS.
PROTECTION
INLET VALVE
CONTROL
SERVICE
RESERVOIR
PISTON
TRAILER SUPPLY
LINE
TRAILER SERVICE
LINE
RELAY VALVE OR
MODULATOR
SPRING
BRAKE
RESERVOIR
TRAILER SUPPLY
LINE
TRAILER SERVICE
CONTROL INLET &
LINE
EXHAUST VALVE
RELAY VALVE OR
MODULATOR
SPRING BRAKE
™
FIGURE 4 - SR-2
TRAILER SPRING BRAKE VALVE
OPTIONAL ANTI-COMPOUNDING
from the control and pressure protection pistons. With
air pressure removed the control piston is moved by the
return spring, control inlet valve closes and the exhaust
passage through the control piston opens. Opening the
exhaust allows air in the spring brake emergency section
™
to be exhausted out the exhaust port of the SR-2
valve.
Spring force above the pressure protection piston closes
the pressure protection valve while the service and spring
brake reservoir check valves close and protect against the
loss of pressure in either reservoir.
FIGURE 2 - SR-2™ TRAILER SPRING BRAKE VALVE
CHARGING ABOVE 55 PSI
SERVICE
RESERVOIR
CHECK VALVE
SPRING BRAKE
RESERVOIR
CHECK VALVE
SPRING
BRAKE
PRESS.
PROTECTION
VALVE
PRESS.
PROTECTION
INLET VALVE
CONTROL
SERVICE
RESERVOIR
PISTON
TRAILER SUPPLY
LINE
RESERVOIR
TRAILER SERVICE
CONTROL INLET &
LINE
EXHAUST VALVE
RELAY VALVE OR
MODULATOR
SPRING BRAKE
™
FIGURE 3 - SR-2
TRAILER SPRING BRAKE VALVE PARK
APPLICATION
SERVICE APPLICATION WITH PARKING BRAKES
APPLIED OPTIONAL ANTI-COMPOUNDING PIPING
(Fig. 4)
With the optional anti-compounding feature of the SR-2™
valve in use, the exhaust port of the SR-2™ valve receives
normal service application pressure through a line
connecting it to the delivery of the trailer service system.
If a service brake application is made while the spring
brakes are applied, compounding of both applications
is prevented by service air pressure entering the SR-2
™
valve exhaust port, fl owing through the exhaust passage
of the control piston, past the open exhaust valve and out
the SR-2™ valve delivery port to the emergency section of
the spring brakes.
NOTE: During normal vehicle operation, with the spring
brakes released, service air pressure will enter
™
the SR-2
valve as described, but will stop at the
closed exhaust valve. See Figure 2.
2
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