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Anyone know anything about compressors?


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I have an old Hydrovane, and my switch leaks air and the electrics are shot, you have to wiggle the wires to make them work.

I have found this:MAchine mart switch

I see the port which connects to my tank, but what does the little one to the right do? My original one only has one port which goes straight into the receiver.



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Learn something every day.

Did not know it was to unload the compressor but that small pipe connects to the non return valve which is where the compressor outlet connects into the tank.

If you don't have that already then I don't know if the switch will still work without the connection. I suspect it will because in it's correct application there may or may not be pressure on that line but the switch will still work.


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Following my logic of how it must work.

Pipe connects to the compressor side of a non return valve into the tank.

Other end of pipe into switch.

I assume when the pump comes on the switch must wait until pressure is detected on the small pipe at which point it closes the pipe so it does not continue to vent pump pressure.

So if left open to atmosphere it will just continue waiting and never close off that fitting but the pump will still be running.

Does that make sense?


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if you don't need the unloader valve connection. As i understand it you only have a screw in pressure switch like the one at mattsavage?


ref 90100

I think machine mart do them but not easy to spot.


ref 000320100

website is a bit sketchy but catalogue describes as a 1/4" bsp 1 port. £16.99 inc



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glad to be of help Roly, the picture wasn't the best, my 4 port one had the same unloader valve and I managed to break the casting on it, so took it to bits to have a look. :rolleyes:

My take on the little valve.

The little valve is an inlet from the back of the 1 way valve (compressor side). once the pump is switched off and the air has gone past the one way valve into the tank, the little valve opens and allows the pressured air that didn't make it into the tank back to atmosphere to relieve pressure on the comressor allowing it to restart not under pressure?

I had to buy another 4 port as my compressor needed a pressure relief, gauge, 3/8" bsp, and tank connection. I forgot the casting was white metal and over tightened the npt/bsp taper splitting the casting.

I now use 1 or 2 wraps of ptfe plumbing tape and not 5 or 6.

I also tried samsonal red which is supposed to be the muts danglies, but is actually useless.


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