Tilt and Trim

When I let my tilt up it eases back down to the trim in about 25 seconds all by itself. What could be causing this and what do I need to do.

[boats.net]

Check the manual release screw on side of motor bracket, it should tight.

I checked it and it is tight.

[boats.net]

One or more internal valves or seats are not holding pressure, system probably needs to removed and rebuilt.

I took it to a marina and they said $1600 to $2200 to fix it. This sounds really high to me. What do you think.