After serious thinking Rick Hughes wrote :
The over pressure valve is not adjustable .. just a plunger spring loaded
against a seat.

Your unit sounds pretty much identical to my own...

Mine blows off its seat and re-seals a short while before the
compressor is shut down by the pressure switch. I suspect the spring
might well weaken over time, so my plan is to try fitting a thin washer
under the spring to see it cures it.

I can't remember all of the details, but the outlet pressure regulator
developed a problem of leaking air - I found the 'rubber' seat seals
had some how managed to turn itself completely around inside the
regulator.

Just to repeat, the pressure switch is adjustable.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk