Tractor Repair: case 1070 three point, piston seals, valve seat

Question
I have a 1976 case 1070 and the three point arms lower when you shut the tractor off. You can stand there and watch them lower . I have a tool bar planter and it sinks fast then lifts continuously all pressures are normal by the guages. I did not run any pressure test. Has plenty of lift power everything else works good.

Answer
  The oil could be leaking past the lift piston seals, or could be leaking through the hitch control valve.  The control valve is on top of the transmission under the seat and can be accessed through the removable cab floor plate.  There is a large spring on the front of the control spool.  There could be a sticking spool in the valve or a bad lift check valve seat.  If everything is OK in the valve, then the lift piston inside the rear end has oil leaking past the seals.