Once hydraulic fluid is hot the truck surges in forward and reverse at low speed only. All pressures check out good. Blew compressed air into port Y and Z (forward and reverse) of hydrostatic pump and air was bypassing through the servo pistons seals. We replaced seals and the truck worked fine...but only for 90 hours.
Found once again that piston seals were bypassing. Disassembled pump, seals and pistons where seals ride are like new. Reassembled the pump and now piston seals hold pressure but I don't think that the issue is permanently solved though.

Any ideas??