My compressor has a switch on it that doesnt let the air pressure go on the pump so that the motor has an easier time when it trips back in under pressure. I got it second hand & the motor has already been replaced, it really struggles to restart & I've been letting the pressure drop right back to stop it burning out.
I've found this on Ebay but with only 1/4" BSP ports how much CFM will it flow? The pipework from the pump to the tank is more like 1/2"
http://cgi.ebay.co.uk/NEW-COMPRESSOR-PRESSURE-SWITCH_W0QQitemZ130125424785QQihZ003QQcategoryZ30906QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

i think you require a non return or one way valve. then the motor doesnt get any feedback from the tank = no presure to stop it restarting.

On my compressor the switch doesn't actually flow any air, it's plumbed into a different outlet on the tank and just detects pressure and switches on and off. There is a one way valve with a small pipe to the switch which bleeds the pressure off the pump so it doesn't start up against any resistance.

The feature you are looking for is called an "unloader valve".

These usually have an orifice that may be plugged on your old one. You may be able to dis-assemble and clean it.

There are no pipes to the switch other than the signal pipe to trip it, I'll have another look at the pipework to see if there's a valve of some kind that I hav'nt noticed but the switch with the pressure release valve on it was how I planned to sort the problem, I'm just not sure if the 1/4"BSP ports will flow well enough.
In theory it should'nt matter too much as the Q/R couplings are 1/4" anyway but you would'nt use 1/4" hose right through the system.

Or is it a leccy fault? What sort of motor? If it has a big fat capacitor to help start it, these can sometimes breakdown.

Hi Owelly! The motor is not the original & does indeed have a big fat capacitor on it.
I'm no electrical expert but the capacitor looks fine & it starts no problem at 40-50 PSI
If you let it trip back in on it's own it just hums, it mabye would start if you left it but thats just gonna wreck the motor. I've only ever let it do it for as long as it takes for me to down tools & cross the workshop. I'm now in the habit of switching it off as soon as it's built up pressure.
A mate has a near identical one but his is a Snap-on branded one & has the switch with the release valve on it, mine is stickered up as an Ingersol-Rand.

The unloader is between the tank and the pump. It will usually resemble a check valve.

The primary purpose of an unloader is to release the pressure from the pump as soon as the pump shuts off. This is the hiss you normally hear after an air compressor shuts off.

The unloader can be built into the head on some cheaper compressors.