The bite function (bucket curl in) on one of our grapples dropped from 3500 psi to 1500 fairly suddenly (according to the operator). I changed the relief valve and that solved the problem but I took the old valve apart and didn't really see what caused it to fail. Any Ideas?
Heat ruins the springs.
It takes only the tiniest particle of foreign material (metal) jammed between valve and seat of a relief valve to dump a lot of oil. Many many years ago, one of my D6C tractors suddenly lost all hydraulic ability. The engine ran, the sound of oil being pumped could be heard - but nothing happened when you pulled the blade or ripper levers, or tried to operate the blade tilt. We pulled the integrated hydraulic system that is mounted behind the dashboard and dissasembled the hyd valve bodies. We found nothing until we removed the pressure relief valve and dismantled it. Between the valve and seat there was jammed a tiny shard of metal, probably no more than 1/8" in dimensions overall. This tiny shard was securely jammed and held the valve open just enough, to dump the entire hyd pump output. Cleaned the hyd valve system thoroughly and reassembled it, and the tractor regained full hydraulics use and power. If anyone had told me that tiny shard would make that relief valve dump the entire pump output, I wouldn't have believed them.
One trick I have used if I suspect a relief valve problem is to back the adjustment almost all the way out (be sure to count the turns so you can get it back) then run the machine at hi idle over relief, sometimes this will clear a piece of debris out of the primary poppet.