BENDIX SD-03-1901 Manuel D’Utilisation

®

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 the reservoir pressure rises above the 

valves pressure setting.  This pressure is determined by 

the force of the spring.

To illustrate the operation of the safety valve, we shall assume 

that the governor cut‑out 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 cut‑out pressure. The opening and closing 

pressures of the safety valve should always be in excess 

of governor cut‑out pressure setting. The pressure setting 

is stamped on the lower wrench flat of the valve.

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.

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 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 one (1) inch 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).

PRESSURE 

SETTING 

150 PSI

EXHAUST 

PORT

BENDIX

®

 

ST-3

™

 VALVE

BENDIX

®

 ST-1

™

 VALVE