"88 760 Turbo- A/C face vents stop working when accelerating. Had a vaccum bellows replaced 2 years ago. Problem is back. There are three vaccum hoses from engine that connect to three ports on the firewall, next to the evaporator pipes. Black, white and grey. The black one does not hold pressure when air is applied where it goes through the firewall into the area behind the blower behind the glove box.
Have replaced the check valves above manifold. Where does the black hose go? When a/c is running, should black hose be holding pressure? If not, what does it indicate. On the inside there are several hoses that meet in a straight line on the front of the blower area. They all seem to hold pressure. Suggestions???