Suzuki trim help (Showa System)

I have a 1996 Suzuki Dt75 with the internal (Showa) trim system common on 1997 and later. the trim works well but recently when i use it and tilt it all-the-way up it wont go back down. the motor pumps but wont catch and suck back down. seems like it may need fluid but i tilted it up and supported it and loosened and re-tightened the manual release valve like the manual says. and it wont take anymore fluid i pumped a bunch of fluid in and hurried to put the cap back on and it seemed to help. thought it was fixed but its not. when it gets stuck up i release the manual releif valve until the motor drops a few inches from fully up and then tighten it and the trim and tilt works fine throughout most of the range. the cap on the reservoir will turn past the threads if i tighten it too much. I guess its just stripped or needs a new gasket maybe. (the thing is that it has always been like that since i have had it and its never given me a problem) is this the problem or do i need to use a different technique when filling the system? please help im a boater on a budget. Thanks for any Help. P.s. I really need a prop 13 x 19 for my boat; some jerk stole my power tech ss prop last week. another setback to getting back on the water. HELP!