My friend has a 1993 Formula that the drive doesn't stay up, it will drop from the trimmed position overnight. Does not have any leaks internally or externally so i assume its a check valve in the pump. Can those be replaced or does the entire pump need to be replaced? Could it be bleeding off somewhere else besides the pump but not leak? Thanks for any advice

my formula did the same thing last year.... tested the pump first..... but it ended up being the pump. You can replace by itself. Think it will come with the tank.

So the entire pump assy needs to be replaced? There are quite a few on ebay, just wanted to be sure before ordering one.

Do they make Viagra for boats?

Reggie Fountain put them in every boat they made.

after you get it fixed, Id leave the drive in a neutral position for storage,

no he didn't. The stringers and transoms are limp as noodles.

Mine was doing the same thing it turned out one of the ball bearings in one of the check valves had a rust pit in it.

Mercury may have a rebuild kit for your pump.
It is not hard to do.