Have one drive that trims up and down, but if I try to leave it up while driving it comes back down. Sounds like a valve is not working, have no external leaks,and the oil level is full???

Just had the same prob on my 93 old boat. Trim pump valves. Call BAM for parts. I had Riggs Marine in Fondulac WI do mine and he had some issues with performing the service bulletin recommendations. Mel said he would save my old parts and send/talk to Oklahoma Mercruiser plant. Glad I didn't mess with it.

Short of that, buy trim pump and ck lift rams to make sure that is not prob.

here's my old thread!
http://www.offshoreonly.com/forums/s...ight=Trim+pump

Some bravo trim cylinders get pitted internally allowing the fluid to bypass the piston seals in the ram and some trim pump valves will leak allowing fluid to bypass them. either will allow the drive to settle down.