Air Compressor troubles

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A few years ago a friend gave me an old Sears 220v, 4hp two stage, belt driven, compressor. I didn't use it until about a year ago. It has ran great until a few days ago. The unloader needle valve on the pressure switch began to constantly blow when under pressure. I ordered a new switch, with unloader, installed tonight...and apparently achieved nothing. The new one does the same. This leads me to think the problem is with in the cylinder head itself. Anyone knowledgeable about this?  I have a larger single stage compressor but prefer to use the old one at night due the the drastic difference in decibel out put.Miller TrailBlazer 251Miller HF-250-1Miller MaxStar 150 STLHyperTherm PowerMax 380 plasmaLincoln PowerMig 180Millermatic 252Miller Diversion 180
Reply:Sounds like your check valve. There should be a check valve on the way into the tank that has a line going to your unloader valve. The unloader is meant to allow the pressure out of the compressor pump but once it starts to dissipate through your unloader valve the check valve closes keeping the pressure in the tank. I had this problem TWICE, both times found a little piece of debris in the check valve holding it open.
Reply:is it doing this once it builds to a certain pressure , or all the time while running.?        thermal arc 252i  -  millermatic 350P -   miller XMT, cp300ts, 30a 22a feeders, buttload of other millers, handfull of lincolns, couple of esabs  -   Hypertherm 1250 G3
Reply:I'd replace your check valve.  I forget what the correct term is, but its where the line from the pump enters the tank.  I saw them for $15-20 at the local Bomgaars a few months ago.I suppose the reeds could be breaking up and chunks getting lodged in the unloader valve, but I kinda doubt it.Last edited by Boostinjdm; 02-19-2010 at 12:23 AM.My name's not Jim....
Reply:Thanks guys for the info. Right after submitting this thread, I found a diagram online. You are correct, it is the check valve at tank entry.  But as usual, I did it backwards. Prior the finding the diagram, I deducted it had to be a check valve failure. I removed the fill line at tank and could see bottom of tank. This, and the fact that the unloader line attached at cylinder head next to pressure line, convinced me the check valve existed internally in the head. I removed head, no check valve.....The check valve is at tank entry but had broken off inside tank, giving a clear view to tank bottom. HA!  Oh well, freshing up the head gaskets won't hurt. New valve and gasket set cost me 50 bucks. Not too bad.Miller TrailBlazer 251Miller HF-250-1Miller MaxStar 150 STLHyperTherm PowerMax 380 plasmaLincoln PowerMig 180Millermatic 252Miller Diversion 180
Reply:I usually cut my own gaskets...  I can't bring myself to spend that much on something I can make with $3 worth of material, a razor blade, and a gasket punch set.My name's not Jim....
Reply:My secret to gaskets is a ball pein hammer.Just go really gently, and keep tapping the ball at all of the corners, and the head will clowly cut the gasket for you.  You just need patience, or you'll dent things.
Reply:what about a rubber mallet..?   i've just used machinist ink/dye....like the hotrodders use to port match on race cars.        thermal arc 252i  -  millermatic 350P -   miller XMT, cp300ts, 30a 22a feeders, buttload of other millers, handfull of lincolns, couple of esabs  -   Hypertherm 1250 G3
Reply:You need something kind of hard.  The hard surface of the ball, against the hard but sharp corners of the casting, have a scissoring action, which is how it works.  A rubber mallet would give you fuzzy edges.  Maybe a harder but safer nylon hammer would work.  Dunno, never tried.Its actually not as bad as it seems.  The gasket paper protects the corner somewhat, and I've done this on aluminum carburetors with no issues.