Power trim problems are messy to diagnose w/o the 91-52915A6 test gage, or a 3000 psi single gage with a male line adapter. First check the reservoir level. It should be within the max/min marks. If not, you probably have a leaking cylinder, and and oil may be present at the piston shafts around the end seals. Also, if your oil is milky white, that seal is leaking.
While the drive is up, crack the up/front line on one of the cylinders at least one full turn and have a helper trailer it up. You should get good flow, and the reservoir level should drop. Tighten this line with the helper running the pump. The pump should labor and go into 2200-2600 psi relief when you do this. Since you're able to trim up, all of this is probably OK. If not, the thermal relief, pressure relief, or poppet valves may be bad or stuck open.
With the up line still open, you and your helper try to force the drive down. If it doesn't, or is very difficult it's a sign that the down pressure valves may be good. If it drops easily, the down pressure relief or thermal relief valves may be bad. If it won't go down, crack a rear/down line and force it down. This will suck air into the cyl., but you can deal with that later. Now tighten the up line and have the helper trim down. You should get good flow, and the pump should labor slightly when it goes into 400 psi relief. If it doesn't, the relief valves are probably stuck open, or failed.
Tighten the up line, with the down line open, and trim back up, and tighten the in/down line. This will purge the air from the up side of the piston. So if it goes back up and stalls at the top, that 1/2 of the circuit is OK. If it will not power itself down, There is a valving problem.
I guessed at your drive at
http://www.marineengine.com/parts/m...hydraulic-pump-oildyne-pump-plastic-reservoir check for your specific pump. It looks like the repair valves are available. $240 for everything if needed.
I bought this last summer to replace my Gen2 pump.
https://www.amazon.com/Reservoir-Mercury-Marine-14336a20-14336a8/dp/B004P3DOWY Not worth diagnosing and rebuilding mine for a new $240 complete pump.
