Several things can caused one clutch pack to drag. If the plates are warped - it will always drag. If any pressure oil is getting to them - it will also drag. Lack of that 20 PSI "lube" pressure can cause much bigger problems.
One question. When you had it apart did you check the lube-oil jet that is screwed into the front of the main output shaft? If it was plugged or damaged - it will cause BIG problems. Often that center bearing assembly will melt or sieze. That is the weakest link in the reverser. Later reversers got larger updated center-bearings. If that bearing is damaged - it will also result in the machine working very hard when in hydraulic reverse. Maybe you were very careful. But most reverser failures I've seen after a rebuild - were caused by a bad oil-jet in that shaft and/or center-bearings ruined because they were not adjusted properly. You cannot adjust them by "feel" like wobbling the front shaft. The reverser must be upside down and end-play at the rear shaft checked with a dial-indicator. Put it a .002" to .004" and it will be good. Do not try to get the front shaft tight enough so it doesn't wobble.