6600 Hydrostatic Drive Problem

I’m having a problem with the hydrostatic drive on my 6600. It seems a little weak and the ground speed is slow in both directions. We use to combine at the high end of second or the low end of third. We are now combining at the high end of third just to keep the same speed. On the road in fourth we can only get about 12 mph out of it. Also when I engage the PRWD the ground speed doesn’t change as much as it use to. Before the PRWD would almost cut the ground speed in half, now it only reduces it by about 10%. I’ve changed the fluid and filter, but that made no difference. The warning light for the hydro appears to be working properly and is not indicating a pressure problem. I’m thinking the problem could be in the relief valves, but I thought it would be strange for both the forward and reverse valves to go bad at once. Any thoughts? Thanks