T0-20 hydraulics - won't stay down

[booger1945]

Recently aquired a TO-20 (1952). Trying to pull 8-foot spring tooth cultivator, but requires constant down pressure on draft control quadrant. When I release the lever, the quandrant and the liftn arms both return to the raised position.
The upper link yoke is rust-frozen so can't adjust that, and dont really want to replace it if not necessary. Any help is greatly appreciated. Thanks, Booger1945