Tractor Repair: tractor, hydraulic pressure gauge, diesel tractor

Question
1972 Ford 3000 diesel tractor, Hydralic lift works but wont lift enough weight.  changed oil & filter, replaced lift top kit & seals, no apparent problem.  Can you help

Answer
does it improve the more throttle you give it
you will need to put a hydraulic pressure gauge on it an see what pressure you are getting from the auxiliary spool most likely the pressure is down and your pump is worn but could also be the relief valve stuck
does the lift go up smoothly or is it erratic