I'm working on a 960 I bought at an auction years ago. When I attempt to raise the three point the arms shake or pulsate on the way up with no weight on them. Does the same with 500lb of suitcase weights but it will lift them. I've swapped the hydraulic piston pump with on off of a working tractor, replaced the orings on both ends of the hydraulic suction and pressure manifold from the pump to the transmission. I've also changed the hyd oil. Still pulsates. I see no oil swirling in the bottom of the case like the relief valve is opening and no oil seeping from the rockshaft valving either. The cam follower pin is about half worn from what I can see. I'm wondering if there is an issue with the valving in the rockshaft. Any ideas?