I have a 1967 Ford 3000 Diesel Tractor - the 3 point hitch hydraulic lift does not work - I removed the lift cover and found that both suction and return line filter screens needed cleaning, the suction filter was really clogged with sludge. I visually inspected all accessable components and reassembled, installing new o-ring seals where indicated. The lift still doesn't work. I did a pump pressure test as prescribed by the manuel and pump pressure was above 2500 psi. Any ideas on what to check next??