How long since you changed your fluid/filters if any? Sounds like a stuck shuttle, bypass or pressure relief valve or blew a seal on the piston. How old is the seal on the lift piston? If the fluid is dirty with water I'd go the valve. If not the piston seal. Otherwise did you have a serious shock load to the system recently when all this started. You could have ruined an otherwise good valve (SS BB with a compression spring holding it in place. That's how I'd spend my time.
I got a 3 pt box scraper hung up on my drawbar on a '74 International 45 hp 464 one time. I could do nothing to the levers to get it to break loose. So I took a sledge hammer and moved it off/away from the drawbar. When it came loose it did something it didn't do before.....forgot what but disfunctional movement. I found the compartment where the hydraulic valves were located and upon inspecting them, one compression spring on a valve had a crack such that the single spring was now 2 short springs in series. I don't know what that valve's function was, but upon replacing that spring things went back to normal.
HTH,
Mark
