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Hi, to add to the previous post, the unloader valve is part of the pressure switch and should be connected to the output port (or an auxillary port) on the cylinder head of the compressor itself. There's typically a tab on the moving part of the contactor that presses on this valve in the off position to bleed pressure in the compressor. There's a check valve between the compressor itself and the tank to keep pressure in the tank from bleeding off through the unloader valve. The unloader valve is intended to keep the motor from having to start the compressor against the tank pressure. It'll take a couple revolutions for the pressure in the compressor to build up to what's in the tank, by which time the motor should be up to speed. Does the motor run OK if you hold the contacts shut? Be really careful & use something non-conductive to push on the contactor!! Some amount of sparking at the instant the points close is normal, hard to say from what you describe whether thats the case or not. Check the oil in the compressor itself & make sure it turns freely. You shold be able to get a replacement pressure switch at any place that repairs compressors or sells commercial ones (not the Home Depot ones). If you don't live in a big city, might be easier to order one. Only thing to watch is that the one you get is rated for at least the horsepower of the motor. I'd suspect the ones on consumer grade compressors are all pretty much the same. Keith
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