It sounds like there's an issue with your control valve but before you do anything else, can you confirm that you properly bled the system after it was installed? To bleed the system you need to have the engine running, then turn the steering wheel slowly to the left lock, then to the right, then to the left, then right, ... at least 5 times or until all the air is bled out of the system. You should also be checking your power steering fluid level during this process to make sure that you don't run it low (that can damage the pump). Also, if it's an original pump, confirm that you used Type F transmission fluid in the system. If you didn't, you need to flush whatever is in there out and fill it with the Type F, then bleed the system.
After you've bled the system, if you still have the problem then you should get underneath and confirm that neither of the little hoses that goes to the control valve is kinked or damaged. If all that checks out OK then you probably have a malfunctioning control valve.
Personally, I would find a different mechanic too. Mechanics these days do not always understand the old systems and being a Ford dealer is not a default indication of competence. If you have any local car clubs join and ask them for their recommendations.