Tilt / trim issues 2002 80 horse

I'm hoping someone has the fix:

The problem is with my 2002 80 horse Yamaha motor, When the motor is trimmed to the highest position it will stick and not lower when using my tilt/trim button. The trim motor makes the noise but the motor stays upright. I have tried to assist the motor by pushing it down but no luck.. They only way to lower the moter is to loosen the screw.. (I'm very careful not to raise the motor all the way up when on the lake)

I would appreciate any help you could give me.. Thanks

Lastly, The screw is starting to show wear and is getting tough to loosen with a screw driver can that be replaced?

[boats.net]

There was a problem with the main valves on earlier F80 and F100 but the 2002 should have newer main valves in it. Check serial number and if it is below these serials

F80TL 1999~2000 (All)
2001 (67GL-350423 and prior)
F80TX 1999~2000 (All)

2001 (67GX-700599 and prior)
Then replace both main vlaves with part number 67F-43802-00-00.
They are number 50 in diagram http://www.boats.net/parts/search/Ya...201/parts.html

power tilt trim tbl

I have a 2001 80hp with the same tbl, also the motor would drift down and I would have to keep re-trimming the motor when underway.
Talked to a service tech. and ordered new valves hope they work.