Our boat has an '89 Black Max 175 which is a little newer but maybe my problem will help a little. The power trim started acting up while on a fishing trip so I couldn't do much about it then but once we got back I got the boat out to the shop and started troubleshooting the wiring. Our problem ended up being the switch in the throttle control, we could only trim down, however it would let us trim up once in a while. All the other switches (at the motor and and the front of the boat) worked fine. I just need to find a switch or throttle control with the switch in it now.
Just a thought to next time it isn't working with the switch you could always bypass the solenoid momentarily with a short piece of wire just to see if it will work that way, if it does your problem is likely electrical. And not with the pump side of the system.