1988 mercury 150 trim raises and lowers fine, If I raise the motor all the way up and leave it in about 15 min. its all the way back down.I see no leaks and have added fluided very little. when running across the lake it will not trim up very far and I am only getting 5000 rpm at 53 mph. any ideas?

It sounds like you have 2 problems.

1. The trim has a leaky seal inside and is why it keeps going down. It could also be some junk in the seal making it leak.

2. The performance is going to require some more info .

a. Is this a new problem or is this boat new to you?
b. What kind of boat is it on?
c. How long has it been sitting before you took it out?
d. has any work already been done to it?

Let us know and we may be able to point you in the right direction.