Trim unit

[Naterj14]

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.