hydraulic pump question

we have tractor with a ccls pump that once you move any of the controls signaling the pump thru the load sensing compensator valve to meet the demand for that circuit up to 2800 psi it will not return to stand by pressure of 350-400 psi, after a barrage of system test the pump was rebuilt even though there was almost no signs of wear, put on a test stand and checked during which time the compensator valve was replaced as it was doing the same thing while on the stand. The replacement was a used one that worked correctly until it got put back on the tractor, so a new OEM one was installed it worked about 6 times correctly but the max pressure had to be lowered, after that adjustment then it started locking in demand not returning to stand by. All three of these valves have had the signal line opened via a line and screw valve connected a tee fitting at the X port of the compensator valve when the pump was locked on demand to insure there wasn't pressure present, even though there was 0 pressure at the X port of the compensator valve the pump stayed on demand until the engine was shut off, part of the time it would still be on demand after restarting the engine. Also if the max pressure valve is adjusted the pump pressure will adjust accordingly. Is it possible to have 3 bad control valves in a row or have I missed something?

"All three of these valves have had the signal line opened via a line and screw valve connected a tee fitting at the X port of the compensator valve when the pump was locked on demand to insure there wasn't pressure present, even though there was 0 pressure at the X port of the compensator valve the pump stayed on demand until the engine was shut off"

You have proven something is not right with the pump.

Cut your loses and aggravation and replace it.

Been there and done that, I'm too tired tonight (going to bed) to type out the long version, short version is the compensating valve provides pressure to a piston to overcome a spring and center the swashplate and take the pump "out of stroke".

If due to internal leaks the piston doesn't move when the compensating valve sends pressure/flow to it the pump will remain "in stroke" even though the compensating valve is providing a "destroke signal/flow of oil" yo the destroke cylinder in the pump..

To answer your last question I'd say no - it would be extremely unlikely to get three bad control valves in a row. Something else is going on.

Have you checked the case pressure on the pump? It should be very low, something in the 5 psi range. The LS pressure references case so if that is elevated for some reason it could mess with the control.