1995 115hp Mercury

The power trim & tilt (PTT) on a 1995 Merc 4 cyl. 115hp. PTT is a single ram with memory piston, round motor, yellow fill plug. The piston rod wiper and seal were old and leaking. I removed the assy. and correctly replaced all the seals and check valve balls in the piston with a kit (about $25) from FSM in FL. Refilled on the bench with ATF type F, fully closed manual release valve and bled system. It worked fine on the bench, but when reinstalled, would only trim up a couple of inches and stop, when under power it would settle all the way back down to bottom. No apparent leaks and reservoir is full at full extension of cylinder. My question is whether to keep replacing, hoping for a cheaper fix or to have a rebuild done for about $550 from FSM with a 2 year warranty? I see some of the parts such as a manifold repair kit are $330. Could it be a bad seal/broken tip on the manual release valve (original nylon one) and replace with a new SS one or bad poppet valve(kit is about $100)? Thank you for your help.