72' Sea Swirl Trim will not go down in all the way.

I have a 1972 Sea Swirl 140HP with Mercruser outdrive. The power trim will raise just fine. when lowering, it will make it about half way and stop. The pump will continue to work. I disconected the rams from the outdrive and tried to opperate them with out the load of the out-drive. again they will travel all the way out without any problem. when I try and bring them all the way back in with down and in. One ram will come about half way in and stop the other ram will then move about a quarter of the distance in and stop then travel back out or up. done this many times and each time the rams will do the same thing but not always at the same point in the travel. once I had one ram come all the way in and the other one half way, but always get one to travel in the wrong direction.

I'm frustrated and ready to pull the plug in the middle of the lake and let some diver enjoy finding a new dive site.

I have checked the fluid, flushed the system with new fluid, manually bled the system, still having the same problem. I thought maybe a bad ram or pump. How do I figure out if it is one or the other or both. Fluid is new and at the proper level.

Any help would be great. The boat is nothing special but it will pull the kids on a tube and we have a great time when it was working right. the thing is great on fuel and really would like to get a few more seasons out of it.

Dan