Hi,

These (small) piston compressors have an 'unload' system that releases the air when it shutsdown.

It needs to do this, because the motor is not strong enough to start up against pressure in the outlet pipe (the pipe going from the compressor to the tank).

When the compressor stops, you will/should hear the air being blown off.

There's a checkvalve right where the pipe enters the air receiver, to prevent air flowing back.

There could be two things wrong in your case:

1. The air isn't blown off.
2. The check valve is leaking.

If the check valve is leaking, the pipe will re-fill with air after the blow-down is over.

Please check on your compressor. if you hear the air blowing of when the compressor stops, the blow-down is working and it's probably the check-valve.

If you don't hear the blow-down, it is probably the blow-down valve. it is located on the pressure switch.

Thanks :)

Cas