I had the same problem with my 1993 aerostar. I took it to ac shop and they had to remove electrically actuated valve in coolant line and replace it with a short piece of flexible hose approx 18" long. The valve is inside the cover at rear unit, now my rear air works just fine. Ford said the valve wasn`t needed. It was redudant. so my problem was solved, of course had to pump down and recharge system.