Trim unit
 

I've got the boat in the drive way right now doing some work on it, and I have noticed when the motor is trimmed up that over night it drops back down about half an inch, no noticeable sign of leaking, also when we traded our old boat for this one our friend said that there was some air in the unit would this cause this to happen, or could it be low on fluid if so how would I check it, its an 88 150 merc.

thos two things could be it, or other things that can cause that situation are the o-rings on the manual release valve are leaking or an internal check valve is not checking properly.

i have the same engine
1st trim the motor all the way up and lock it in
2nd on the left side theres a big black nut attached to the pump the nut[i believe] has a slot that looks like a screw driver should fit in but you dont use a screw drive use a socket
3 remove the nut fill with quick silver trim fluid put the nut back on and your done fill to the top
MAKE SURE THE MOTOR IS FULLY IN THE UP POSITION OR YOU WILL BE WEARING THE FLUID ON YOUR CLOSE
and hopefully thats would fix it also theres a nut on the right side that if you losen it you can raise and lower by hand maybe that nut is loose check that aslo if its loose it wouldnt hold pressure and fall itself lets us know how you make out