Tilt/trim Problem

Seem to have a problem with my tilt/trim. It may be as simple as low fluid or it could be something else. The motor seems to work just fine, however when I press the tilt/trim button the tilt motor goes but will not raise the motor. Should I be able to tilt the motor up by hand, because I can and I thought when full with fluid I wouldn't be able to. There is no fluid leaking anywhere. What is the process to fill the tilt/trim unit? This is a 1986 40 HP Yamaha made Mariner.

Thanks!

[boats.net]

Raise the motor up all the way and fill power trim reservoir with fluid.

That is the screw that I remove with the allen wrench right?

The screw on my unit is a blade screw and is located on the resivoir. The hex head screw on the base of the unit serves no function after the machining of the unit is completed. Also, you may want to check the manual lift screw to see that it is not open. It is important to not that it is a reverse thread--I learned that one the hard way--

Thanks all! I will check on this when I get home this afternoon. Just to note the hex head screw is on top of the unit right near the piston rod not the base of the unit.