Who told you your check valves were eroding, or did you tear it down and confirm this? Hydraulic fluid usually doesn't cause erosion, but cavitation does. Erosion can also be caused by minute particle circulating in the fluid that are smaller than 10 microns, which is the filter you have. They have filters that go as low a 1 micron. Did PT give you any feedback on the pump you returned. Were both check valves eroded. You have I believe a check valve in fwd, and one in rev.
Again, a hydraulic gage would help determine the problem. If you know what the pressure should be or the operating pressure range, then you have a baseline. If what you say is true, and the check valves were opening, then you would be generating a lot of heat, because I believe the relieved fluid is routed back to the front of the pump. and and it also being a closed loop system, and you know that heat reduces viscosity