it could just be the OD relay...at its age though it could be the seals. i've done that job, while not particularly hard its and undertaking. and the OD didn't work when i put it back together either. there were broken pieces inside of it...i can't remember exactly what. i know the metal bars the pistons engauge against were bent all out of shape (which i tried to fix). it would try to engauge, then slip real bad. the mechanic is probably thinking its something electrical (which is easy) and looks to make a quick $300.