JD 2755 Hyd. System

[Graper00]

I recently bought a JD 2755 at an auction. The 3point lift works good going up but will not come down with out me pushing down. It has a return control valve on it, with it all the way open it goes down better but still needs weight. Also when lift gets out of the all the way up position the line coming from pump jumps and pulsates like there is air in line. I have worked tractor for appro. 2 hrs. hoping maybe air was in system and would find it's way out but still same problem. Any ideas?