Trim pump won't tim down

[pofarm]

I just installed a new Arco oildyne replacement trim pump. It will trim up ok but won't trim down. It pumps fluid out of one port in up and the other in down like it should. If I run it long enough in down, it will raise the motor for some reason. I have tried bleeding the system, but without it going down it is almost impossible to do.
Any ideas what may be causing my troubles?
Thanks

[H2OPERF]

Does the motor sound loaded in the down mode like its building pressure or is it just cavitating?If it sounds like its pumping fluid in the down mode the seals on your piston are bad and its bypassing fluid to the up side making the motor go up.If the motor is running fast no load its probably just air in the line.Dave

[pofarm]

The motor takes a while to load, but when it does it trims the engine up. I am thinking that maybe the cylinders are bypassing because if I loosen either line at the cylinders and run the pump in either direction there is pressure coming out of the loose fitting. It seems odd to me that it will trim up but not down, though. I would think that with the pressure bypassing in the cylinders like that, the weight of the engine would hold it down and it wouldn't trim up.
This is really confusing and frustrating me. I can usually figure these things out, but this one has me stumped.
By the way, I checked the old pump, which would trim it both up and down, and there was no pressure when running it in down. It seems that the weight of the engine is forcing it down.
The reason I am working on it is because it has the old system that has the reverse lock solenoid. If I engage the reverse lock by turning the T handle in the valve body clockwise, like it should be, the trim won't work at all (motor runs but no pressure). I was told that I could replace it with a newer style Mercruiser pump, which I have, and now am having the problems that I have posted.