90 Yamaha Pwr. Trim Leakage

[Bennie Benoit]

My 90 Yamaha when in the raised position will
eventually lower itself. It won't stay up in the
position as it was left. Usually overnight it will
go down. I have checked the fluid level and it
is full. I have tried the adjusting screw on the
right side and it is unscrewed to the max. Turning
this screw in will make it release the pressure and
lower the engine. The engine raises and lowers
properally and appears to operate ok. Does any
one have any suggestions?

[boats.net]

One of the internal check valves are leaking but would have to rebuild it to replace valves and valve seats.