Do the arms drop if you cut the engine. They will drop slowly over say a few minutes under heavy load but if only a minute,that could indicate worn or broken lift cylinder piston rings which leaks oil back into the tx sump when the engine stops and will reduce pressure
Could also be missing or damaged seasl on the lift tube that comes up from the pump to the housing on top of the tx behind the gear sticks and secured by 2 bolts. There is an o ring and nylon support ring on each end of the tube.
Could also be blocked inlet filter but that would normaly slow the lift down, not reduce power
The other would be baddly worn pump or worn / damaged relief valve on the pump. You need to lift the top of the box off to get at this although filter can be reached through he PTO side plate but collect the oil to reuse if clean first.
of course, could also be issue with the draft or lift control links causing the pressure valve to stay partly open to return
pm me your email and i will email you the drawings
Nige
Ps just had ours apart although i have now a funny issue with ours. Loads of lift power on the 3 point but not enough at the top of travel for the FEL until I switched it to the aux2
I posted it on here
Good luck