My GT 2550 has been great but is getting harder and harder to move in reverse. When new, 14 years ago, it was slow but now it crawls. I've changed the Hydro-oil on a regular basis and it still creeps. I read somewhere that the Hydro pump was the weak point of this mower, is that true? Is there a fix other than changing it out?
Hydro Gear makes a rebuild kit, I'll give it a 50% chance of working, when you get inside if the piston bores are scored it probably won't work.
If it were me I would take pump apart first and give it a good look before buying rebuild kit.