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Unloader Valve


Remove the unloader valve from the reservoir.

The valve is dismantled by removing the screwed caps retaining the filter and the check valve. Unseal and remove the top cover containing the piston spring adjuster; care is necessary during this operation as the spring is under compression. Take out the piston spring.

Remove the cover below the bellows.

Hold the piston from rotating by means of the two slots at the top and use a 1/8 Whitworth box spanner to unscrew the set-screw securing the bellows to the lower end of the piston.

Draw out the piston from the top and the bellows from the bottom of the body. Check that the piston works smoothly in its bore without any sticking. Lap the piston valve lightly on to its seat using metal polish.

Check that the piston spring and the check valve spring. If any signs of distortion or corrosion are present, the springs should be renewed.

Carefully examine the bellows for deterioration or distortion.

Examine the check valve rubber. This should be wiped with a clean rag or if damaged renewed. No attempt must be made to clean the seat with abrasive or a cutting tool.

Wash the felt pads of the inlet and exhaust filters in paraffin.

The various jointing gaskets and washers is in other than perfect condition must be renewed.

Assemble by reversing the procedure for dismantling, noting the following points:-

Lightly oil all moving parts.

A copper washer is fitted under the head of the set-screw securing the bellows to the lower end of the piston and another between the bellows and the piston.


Charge the reservoir by running the compressor or by admitting air from an air line. The unloader valve should be adjusted to unload when the reservoir pressure gauge indicates 95 psi.

Slowly lower the reservoir pressure by opening the drain plug or cock. The unloader valve should cut in again at 75 to 80 psi.

Close the drain plug or cock and charge the reservoir to 95 psi, checking meanwhile that there is no escape of air through the exhaust silencer, or from any of the joints surrounding the unloader valve.


The unloader cut-out pressure is adjusted by slackening of the locknut and turning the adjusting thimble clockwise to increase and anti-clockwise to decrease the pressure. When increasing the pressure it is desirable to screw down the thimble just beyond the desired point and then turn it back, so avoiding any twisting of the spring that will affect the setting. Tighten the locknut and recheck the setting.

There is no adjustment for the cut-in pressure, and if this is low, the unloader valve must be dismantled again and checked. It is important that the reservoir pressure does not fall below the cut-in pressure  but it does not matter if the actual cut-in pressure is a little higher.

Unloader Valve


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