Sterndrive doesn´t keep trimmed

Hi, i am new in the forum and found it searching for a answer on google about my sterndrive.
I have a 5.0 FL Cobra Engine with a SX Sterndrive (Cobra).
I found that the sterndrive won´t keep "up" when i trimm it up, in use or with the engine stopped, it goes to the lowest level after a few minutes.

I am calling my mechanic tomorrow but i would like to know what could be causing this.

Also, i´d like to know if i can damage something else if i use my boat with the lowest level in my trim all the time...

Thank you very much and sorry with my poor marine lexis (i am from a hispanic language country)

Make sure the manual release valve {located on the pump housing} is tight {clockwise} if equipted. modle numbers needed.. If the vale seat is good and problem still one of the trim clyenders is most likely your problem. thes cylenders can be rebuilt most IO SHOPS. FAIRLEY SIMPLE REPAIR

I would also check for a leaking hydraulic line inside and outside the boat.