1999 C90 Yamaha trim leaks down

[<Patrick>]

Under full power, my trimmed up motor will leak down from the "sweet spot" in about 10 seconds. Occasionally, it doesn't leak down at all and works fine. I suspect I have a faulty o-ring or maybe some kind of weak return spring in a valve that regulates flow of hydraulic fluid. I have no problem buying the parts necessary that are likely to eliminate the problem. Does anyone have some guidance on which part of the power trim assembly the potential leaky parts might be??

I have a copy of the parts diagram for the trim/tilt assembly.

Are the valve parts most likely to be leaking on the motor or reservoir side of the trim assemby?

I look forward to any advice you kind people can provide.

[gander]

Patrick, Have you come up with an answer on the trim leak down? I just noticed my 90 occasionally does the same thing.