Have a 5 hp Copeland from the1980s old but still working. The oil presser switch tips out. other tecs have worked on it over the years.The pump has been changed,valve plate replaced (not holding vac.),presser switch replaced.some oil was added. still tip out Now I am called. The unit first started with 502, about 95 they convert to mp 80. He put in a new 10 door and the tec converted to r408. Where do i start looking i normally work on small stuff.