Servex air compressor

A little bit off topic, but I purchased an old 1950's servex compressor with a 5HP 3 phase motor about 15 years ago. Until this week, I never got around to using it as I only have single phase and a 5 HP motor was too expensive. However my compressor has just about died (takes 18 minutes to get to 100psi) so I bought a chinese 5HP motor, some wiring and switches and set it up. All works well and only takes 2 minutes to get to 100psi. The only problem I have is that it leaks air slowly back through the unloader valve. It is not fitted with check valves so I don't know how it wouldn't leak. It all looks to be original, so I can only assume that it didn't previously leak and it was still being used when I bought it. Also, the unloader valve (picture below) was only partially screwed into the crankcase with the holes in the side on the outside of the crankcase. I'm sure someone on the forum has probably had some experience with these, so any comments appreciated.

The unloader valve is operated by centrifugal weights in the dome at the end of the crankcase. There should be a locknut on the unloader valve, you screw the unloader valve in or out so it unloads when the compressor is not up to speed. If you remove the elbow from the valve you can remove the needle valve and check no rubbish under valve.

If however there is air leaking from the valve when the compressor is stopped, then it indicates an exhaust valve in the compressor is leaking. In mine they are flat disks, and I have polished them up on an oil stone and also turned them over. If it has been sitting the valves may be rusty and the light weight valve springs may have rusted.

I have a number of old Servex compressors, single, twin and V twins.
Paul
Paul

Thanks, mine is a V twin, so I will isolate which cylinder is leaking and remove the head for a look see.

The later Servex compressors,especially the 25cfm v twins lost the unloader,and relied on the motor overcoming the compressor at startup........Incidentally,the ones with a small crank,use sideplate Holden bearing shells....This is all the later ones....They all rely on the outlet valve in the head plate to work as the sole non return valve.....Repco were the agents for many years,and used to carry the valve kits.....but the valves can be made by cutting up feeler strip......L.S.Starret brand is tempered hard steel.