Bendix Commercial Vehicle Systems ST-3 User Manual

®

Bendix® ST-1™ & ST-3™ Safety Valve

EXHAUST

PORT

PRESSURE

SETTING

150 PSI

™

ST-1

VALVE

DESCRIPTION

The safety valve protects the air brake system against
excessive air pressure build-up. The valve consists of a spring
loaded ball valve subjected to reservoir pressure which will
permit air to exhaust reservoir pressure to atmosphere if
reservoir pressure rises above the valves pressure setting,
which is determined by the force of the spring.

OPERATION

T o illustrate the operation of the safety valve, we shall assume
that the governor cutout pressure is set at 125 psi. A safety
valve with a setting of 150 psi could then be used. Should
system pressure rise to approximately 150 psi air pressure
would force the ball valve off its seat, and allow reservoir
pressure to vent to atmosphere through the exhaust port in
the spring cage.

When reservoir pressure decreases sufficiently , the spring
force will seat the ball check valve, sealing off reservoir
pressure. This would occur at approximately 135 psi for the
150 psi valve. It is important to note that the desired pressure
setting of the safety valve is determined by the governor
cutout pressure. The opening and closing pressures of the
safety valve should always be in excess of governor cutout
pressure setting. The pressure setting is stamped on the
lower wrench flat of the valve.

ST-3

VALVE

™

SD-03-1901

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

OPERATING AND LEAKAGE CHECKS

OPERATING TEST: With air pressure in the system, pull
the exposed end of the valve stem removing the spring load
from the ball check valve. Air should exhaust from the valve’s
exhaust port. Release the stem, the air flow should stop.
Failure of valve to pass operating test would indicate the
valve should be disassembled, cleaned and rebuilt. (See
“Disassembly and Assembly” section). If adjustment is
necessary, see “Adjustment” section.

LEAKAGE CHECK: Coat the exhaust port with soap
solution. A leakage of a 1” bubble in 5 seconds is permitted.
Excessive leakage indicates dirt in valve, faulty ball valve or
seat. Valve should be disassembled, cleaned and rebuilt.
(See “Disassembly and Assembly” section).

REMOVING AND INSTALLING

REMOVING

1. Block wheels or otherwise secure vehicle and drain
reservoirs.

2. Using wrench flat closest to reservoir, unscrew valve from
reservoir.

INSTALLING

Normally, the safety valve remains inoperative and only
functions if for any reason reservoir pressure rises above the
setting of the valve. Constant “popping off” or exhausting of
the safety valve can be caused by a faulty safety valve, faulty
governor, faulty compressor unloading mechanism, or a
combination of any of the preceding.

Safety valve should be installed in same reservoir that
compressor discharge line is connected to. Install in a
convenient location in a top port of the reservoir. If safety
valve is installed horizontally , exhaust port should point down,
stem of the valve should face rear of vehicle.

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DISASSEMBLY: ST-1™ ADJUSTABLE VALVE

1. Clamp lower wrench flat in vise (flat nearest pipe thread).

2. Using upper wrench flat, unscrew lock nut. Unscrew and
remove spring cage from body of valve.

3. Remove ball valve, spring and release pin from spring
cage.

DISASSEMBLY: ST-3™ NON-ADJUSTABLE VALVE

1. Clamp spring cage in soft jawed vise.

2. Using wrench flat, unscrew body from spring cage.

3. Remove ball valve, spring and release pin from spring
cage.

CLEANING AND INSPECTION

Clean all parts in mineral spirits. Inspect all parts. All parts
not considered serviceable should be replaced with genuine
Bendix replacement parts.

ASSEMBLY: ST-1™ ADJUSTABLE VALVE

1. Place the ball valve in body.

2. Install spring and release pin in spring cage with adjusting
screw.

3. Position the release pin overball valve. Screw body with
ball into the spring cage. Tighten securely.

4. Adjust for proper setting (see “Adjustment” section).

ASSEMBLY: ST-3™ NON-ADJUSTABLE VALVE

1. Install spring, release pin in spring cage.

2. Position ball valve in body and screw spring cage onto
body.

3. Hold spring cage in soft jawed vise and tighten body
securely .

TO RAISE PRESSURE SETTING

1. Loosen lock nut.

2. Turn adjusting nut clockwise to obtain correct pressure
setting.

3. Tighten lock nut.

TO LOWER PRESSURE SETTING

1. Loosen lock nut.

2. Turn adjusting nut counter clockwise to obtain correct
pressure setting.

3. Tighten lock nut.

TESTING OF REBUILT SAFETY VALVES:

Perform operating and leakage checks.

ADJUSTMENT OF SAFETY VALVE

NOTE: The ST-3™ safety valve is not adjustable.
The pressure setting of the safety valve is stamped on the

cover wrench flat (closest to the pipe thread). The vehicle
manual may also provide the proper pressure setting. If
setting is not known, determine governor cutout pressure
setting and adjust safety valve so that the safety valve closes
at a pressure setting somewhat above governor cutout
pressure setting (See “Operation” section).

RELEASE

ADJUSTING

NUT

SPRING

CAGE

BALL

VALVE

PIPE

THREAD

SPRING

CAGE

BODY

PIPE

THREAD

PIN

LOCK

NUT

SPRING

BODY

™

ST-1

SAFETY

VALVE

RELEASE

PIN

SPRING

BALL

VALVE

ST-3™ SAFETY

VALVE

To adjust, the safety valve must be connected to an air
system with air pressure in excess of desired setting. It is
important that an accurate gauge be used to check pressure
settings while making adjustments.

WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID
PERSONAL INJURY OR DEATH:

When working on or around a vehicle, the following
general precautions should be observed at all times.

1. Park the vehicle on a level surface, apply the park­ing brakes, and always block the wheels. Always
wear safety glasses.

2. Stop the engine and remove ignition key when
working under or around the vehicle. When work­ing in the engine compartment, the engine should
be shut off and the ignition key should be removed.
Where circumstances require that the engine be
in operation, EXTREME CAUTION should be used
to prevent personal injury resulting from contact
with moving, rotating, leaking, heated or electri­cally charged components.

3. Do not attempt to install, remove, disassemble or
assemble a component until you have read and
thoroughly understand the recommended proce­dures. Use only the proper tools and observe all
precautions pertaining to use of those tools.

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