After my Hitachi ec12 compressor pumps to maximum pressure it shuts off and immediately starts dumping air out of the part name "Joint" which is item 60 in the parts list. It will dump until the motor tries to start again which it doesn't so the motor starts single phasing. I removed the joint and looked at the pin but couldn't see in the body of the pressure switch. The pin looked as though there was no wear on it.

This problem is caused by either the starter capacitor or the in-tank check valve. Ez to check. With no air in tank, start compressor; if motor hums or has difficulty suspect capacitor. If motor starts, partially fill tank and unplug the power. Check for air leaking at the pressure switch or air coming out of the air inlet (remove the air filter). If air leaking, replace check valve. If no air leaking, again let out all the from the tank and connect power. Allow the compressor to fill and shut-off. After the compressor fills, again listen for air leaking from the pressure switch unloader valve. If air continues to leak from unloader, replace the check valve. Good luck

SAE 5W50 and if available use full synthetic.

This link will take you to the EC12 owners manual if you would like.

http://www.hitachipowertools.com/product/files/pneumatics/compressors/EC12__OM.pdf

http://www.ereplacementparts.com/



Number 63 but go to this site for manual.

I have the same compressor with the same problem. I have temporarily eliminated the thermal reset with a piece of 12 gauge hard wire and it runs just fine. However, I do want to replace it before the warm weather come to the north. I tested the draw at 15.5 amps and the reset was a 17 amp. I am told by someone with superior electrical background that I need to install a 20 amp thermal reset in it's place. I have found that mine runs best when plugged directly into a 20 amp circuit and when using an extention cord I have a 10 guage cord that will carry the load needed to run the EC12. I have also changed to 100% synthetic oil.

sounds like the pressure switch is fused shut, more than likely it needs to be replaced, most single stage pumps should shut off at 125 psi max.

how do you know one tank drops to zero unless you have a pressure gauge on both tanks sounds unlikely unless you are looking at the gauge on the pressure regulator

You have to manually adjust the pressure regulator. Make sure drain is completely closed. The one thats on the bottom of the tanks

well make sure air pressure is not too high. if not just replace it any air supply house carries them

There should be a pressure switch adjustment under the cover of the switch remove the cover and reset.

The burst of air you hear as the compressor shuts off is the cylinder unloading so it will not have to restart against a high pressure load. If the unloader is electrical it would explain the hum that you hear. This could be a normal function.