We have an EC79 Hitachi air compressor. The pressure relief is supposed to stop automatically when full, but this one turns ON and lets the air out. Once it opens, we have to turn the Hitachi off.

SOURCE: champion vr3-8 auto drain blowing out when compressor turns off

IS YOUR TUBING CLOGGED, PINCHED OFF OR MAYBE DIRT-DOBBER NEST IN IT. THE WATER AND AIR IS SUPPOSED TO COME OUT THE SMALL TUBE POINTING DOWNWARDS BUT SOME EXCESS AIR MAY COME OUT THE SILVER VALVE ON TOP. THIS IS THE EXCESS AIR WHEN IT IS UNLOADING AND THE SILVER VALVE IS BASICALLY A TIMER. YOU CAN ADJUST THE DURATION OF THE BLOW DOWN TIME BY LOOSENING THE THIN JAM NUT AND SCREWING IT IN OR OUT. DO NOT BLOCK THE SMALL TUBING OR PINCH IT SHUT, YOUR COMPRESSOR CANT UNLOAD AND WILL START HARD. DONT REPLACE THE WHOLE DRAIN BECAUSE THIS AUTO DRAIN TAKES A REBUILD KIT THAT COSTS AROUND 20.00 TO 25.00 AND IS VERY SIMPLE TO REBIULD. YOU CAN CONTACT ME FOR PARTS, PRICING AND INFO AT [email protected] OR WWW.CANDAEQUIPMENT.NET

SOURCE: The compressor relief valve keeps popping, compressor runs

What pressure is it popping off at? You might have to adjust the pressure switch to a lower setting.

SOURCE: Dewalt 4 gal air compressor will not build pressure in take.

The bleed valve is an unloader of sorts that allows the compressor to start with-out a load. It closes when the pressure in the airline reaches about 20-25lbs with good volume. If air volume is not sufficient, the valve will not close. To solve the problem lets do a few tests. A long cord will reduce power and reduce air volume. Try powering the compressor with-out extension cord. If bleed valve closes when you do not use a cord, the compressor is not getting enough voltage, use shorter cord of the 12 gauge quality and more hose to reach work. If valve still does not close, check for air leaks on all fittings including around pressure switch and drain valve with soapy water. Repair as necessary and retest. If no leaks were found, temporarily plug bleed valve port with screw/bolt and run compressor till set pressure cut-off while counting minuites/seconds. The compressor should fill and shut off within three minutes or so. If it takes an unusually long time to fill, then the reed valves /valve plate is worn and leaking pressure back into cylinger. Leaking valves will usually result in very hot discharge airline/air. Reinstall the bleed valve after test. Parts and partslist are available at www.toolpartsdirect.com . Good luck on the repair and email if you have other questions.

SOURCE: install reed valves hitachi air compressor need pics

I believe in your model of air compressor, when you take apart the head portion to your compressor you should have four reed valves. When you have it apart, there should be two "grooves" on the bottom "plate" and two "grooves" on the top plate (where the reed valves go). the bottom part is easy (as you should already know) to set in the reed valves, but as i have also found out, as you probably have, that its a little more difficult to line up the other reed valves to thier "grooves" on the upper "plate". What you do to make sure that they stay in place is dab a little oil or grease on the reeds and place them in the upper "plate grooves" and carefully set the upper plate back in place where it goes on the compressor. It will hold them in place long enough to install it on the compressor and the small amount of grease that was there will quickly be burned up with no effect to the opperation to the machine.
Sorry, long winded explanation. hope this helps and if it didnt get to you soon enough i hope it helps out in the future. :-D

SOURCE: My Hitachi EC12 builds pressure, shuts off, then air escapes

the problem is in the check valve where air enters the tank.when leaking it lets air pressure up to the air piston causing the motor to stall upon restart.replace check valve.average cost of part 15 to 30 dollars.simple fix