Bendix SR-4 SPRING BRAKE CONTROL VLV User Manual

Bendix® SR-4™ Spring Brake Control Valve

#2 RESERVOIR

CHARGING

PORT

SERVICE PORT

#1

RESERVOIR

CHARGING

PORT

FIGURE 1A FIGURE 1B

DELIVERY PORTS (4)

TRAILER SUPPLY

PORT

TYPE 1

#1

RESERVOIR

CHARGING

PORT

DELIVERY
PORTS (4)

SD-03-4514

#2 RESERVOIR

CHARGING

PORT

SERVICE PORT

TRAILER SUPPLY

PORT

TYPE 2

FIGURE 1 - SR-4™ SPRING BRAKE CONTROL VALVE TYPE 1 & TYPE 2

DESCRIPTION

The SR-4™ valve is provided in two different confi gurations.
Type 1 (Fig. 1A) incorporates a nipple mounted PR-3™
valve. Type 2 (Fig. 1B) has the function of the additional
PR-3™ valve integrated into the main housing. There is
no difference in the operational performance of the two
designs.

The SR-4

™

trailer spring brake control valve is similar
to the SR-2™ valve. It differs in function by utilizing both
reservoirs for normal service braking but reserving suffi cient
air pressure to provide the required spring brake release in
the event of a service system failure. As seen in Fig. 1, the
complete assembly includes an SR-4™ valve sub-assembly

™

similar in appearance to an SR-2

valve. The SR-4™ valve
subassembly is an SR-2™ valve modifi ed by the addition
of a check valve in the spring brake reservoir fi tting which
permits air to pass from the reservoir to the spring brake
inlet valve but not in the opposite direction. AT NO TIME
SHOULD AN SR-2™ V AL VE BE SUBSTITUTED FOR THE
SR-4™ V ALVE SUBASSEMBLY. The SR-4™ valve may be
identifi ed by the metal tag affi xed to the cover by one of
four cap screws. Porting is as follows:

• 1/2 inch NPT or 3/4 inch NPT spring brake supply port
for reservoir mounting

• 4-3/8 inch NPT delivery ports

• 2-1/4 inch NPT reservoir charging ports

• 1-1/4 inch NPT trailer supply port

• 1-1/4 inch NPT service port (optional)

OPERATION (FIG. 2)

CHARGING

Air from the trailer supply line enters at the trailer supply port
and depresses the control piston "D", opening the spring
brake inlet valve "E". It also is conducted to the cavity under
the pressure protection piston "F". When air pressure builds
to approximately 60 p.s.i. beneath the pressure protection
piston, the piston moves against the force of the pressure
protection spring and opens the pressure protection inlet
valve. The air pressure now fl ows past check valve “A”
into the #2 reservoir and past check valve “B” through the
open spring brake inlet valve and into the spring brake

1

RES.

#1

RELAY VALVE

PRESSURE

PROTECTION

INLET VALVES

PRESSURE PROTECTION

PISTON "F"

CHECK VALVE "G"

RES.

#2

CHECK VALVE “C”

PRESSURE PROTECTION

PISTON SPRINGS

SERVICE LINE

ANTI-COMPOUND

LINE

PRESSURE PROTECTION

PISTON "F"

CHECK VALVE “A”

CHECK VALVE “B”

ANTI-COMPOUNDING

CHECK VALVE

SUPPLY PORT

TRAILER SUPPLY LINE

SPRING BRAKE

INLET VALVE "E"

SPRING BRAKE

FIGURE 2 - SR-4™ VALVE CROSS-SECTIONAL SCHEMATIC

units. Air is prevented from fl owing from the SR-4™ valve
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.

P ARK 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

CONTROL PISTON "D"

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.

TRAILER SERVICE LINE

ANTI-COMPOUNDING

CONNECTION

™

R-12
RELAY
VALVE

SINGLE CHECK

VALVE

TRAILER SUPPLY LINE

FIGURE 3 - SR-4™ VALVE ANTI-COMPOUNDING PLUMBING

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-4™ valve should be inspected every 6 months or
1500 operating hours, whichever comes fi rst, for proper
operation. Should the SR-4™ valve not meet the elements
of the operational tests noted in this document, further
investigation and service of the valve may be required.

SERVICE AND LEAKAGE TESTS

Check the tractor dash gauge against a test gauge known
to be accurate prior to performing these tests. Connect
the tractor air lines to the trailer on which the SR-4™ trailer
spring brake valve is to be tested. Block all wheels or hold
both vehicles by a means other than air brakes.

1. Install two separate test gauges or one dual test gauge
with one line to the #1 reservoir and the other line to the
#2 reservoir. Build the tractor and trailer to full system
pressure by placing the trailer supply valve in the charge
position and the parking control valve in the "brakes
released" position.

RES. #1

RES. #2

Note: As system pressure reaches approximately 60

p.s.i., the #2 reservoir and the spring brakes
should build up to approximately 60 p.s.i. before
the #1 reservoir begins to charge (see Fig. 3).
When full system pressure has been reached and
the spring brakes fully released, it is acceptable
to have a slightly lower pressure reading in the
service reservoirs than is registered on the dash
gauge. Apply a soap solution to the exhaust port
and the vent. Leakage of a 1" bubble in 5 seconds
is permissible.

2. Place the trailer supply valve in the exhaust position;
the spring brakes should apply. Disconnect the trailer
supply line and soap the hose coupling to check
for leaks. A 1" bubble in not less than 5 seconds is
permissible.

3. Reconnect the trailer supply hose coupling and recharge
the trailer system. The spring brakes should release.
Shut off the engine, leaving the ignition on and open the
drain cock on the #1 reservoir. The tractor air system
should bleed down to approximately 55 p.s.i. with low
pressure, indication occurring at or before 60 p.s.i. The
#2 reservoir on the trailer should also bleed down to
approximately 55 p.s.i. but the spring brakes on tractor
and trailer should remain released. After the system is
stabilized, leakage at the open drain cock in the trailer
should not exceed a 1" bubble in 5 seconds.

3