Another non-electrical won't trim down pre alpha

[matthysone]

Thinking my problem may be a check valve in the pump. Can easily trim up the drive on this 1980 Mercruiser 470. Try to trim down and no movement but pump is running. Pulled the hydraulic lines, both down and up, from cylinders and fluid pumps out as you would expect i.e. fluid pumps from down line when trim down is activated, same for up. Blead system per manual. While bleeding noted that cylinders worked properly going down. Tighten everything up and down no longer works. Hmmm? Loosened the "up" line on cylinder and again down would work as it expelled fluid into catch pan. Therefore it appears the system is locked as something is preventing the fluid from making a round trip back to the reservoir. Anyone have some insight? I also made sure fluid was making it through the reverse lock valve which it would have to anyways to get fluid at the cylinder down line.

The manual talks of "hydraulic pressure thus created moves a slide valve which unseats a check ball and lets fluid return"

This sounds like my issue. Is the pump and internal valves serviceable?

Any other trains of thought I'm missing?