"My harbor freight compressor high pressure release valve was opening-releasing pressure at about 110 psi even though the compressor is rated to 125 psi.
I found that I could remove the valve and unscrew it's top with it held in bench vice and used two screw drivers and a hammer to tap the top brass cap counter clockwise.
Inside is the spring that is rated to probably 140 or so psi but because it's lost come of it's tension, (so) no longer holds the valve shut to that pressure.
I simply put a screw driver in a vice and inserted the spring end into the driver and used another driver in my hand on the other spring end and forcefully stretched the spring and noted it was now a bit longer.
Reassembled the valve with the rejuvenated spring and problem fixed."
And this from me... Jim's idea of how to fix a pressure relief valve is a good one. The only concern I would have is if you stretched the spring too far, it would increase the shutting force on the piston inside the PRV. As a result the pressure at which the pressure relief valve let go might end up being higher than what is safe.
If you do undertake this method of fixing a pressure relief valve, I would certainly suggest that you test the new cracking pressure to ensure that the PRV would open at a safe pressure level for your compressor.
Thanks for this, Jim.