2N Hydraulics ???

From the sounds of this I wonder if it isn't the control valve or the linkage. I know this is probably an insult but I would also check to make sure the PTO is engaged. I bumped mine on my 2N and it took a fair amount of cussin' before I realized I had hit the lever and took the pump off-line. If it worked that well once, it is likely not the piston but you didn't say anything about the cylinder. If you put a new piston in an old cylinder, the scoring might be bad enough to tear the o-ring and cause bypass. The lift should still function somewhat but would leak down quickly.

Look for something simple first.

Good Luck

Do you have something like this little knob (the one just in from of the seat support)? This is an after market add-on that functions as a valve between your cylinder & pump that allows for auxilary hydraulics. But more important to your question, in my case if I don't have it up my 3PH does nothing.