Vacuum Check valve
Another not so common problem is the vacuum check valve failure, You could blow and suck through the valve both ways. This valve is hooked to the black line in the engine compartment side of the firewall behind the coolant tank. This happened on my daughters 2002 xlt 255000 miles. She was loosing air flow out her front vents while going up hills or under hard acceleration. Vacuum check valve had failed. When she went to removed the valve to test it the hose snap of at the firewall grommet. Now the broken plastics line was pretty easy fix just had to get the hose that broke off out. The hose is glued into a connector that joins two hoses together the connector is also is glued to the grommet. Just used a small screwdriver and worked around the hose and grommet to break the glue loose. She cut the small black line in the cab and pushed it through the large rubber grommet, the one with the two lines going through it. Then was able to used the rubber connector that goes to the check valve to hook to the black line she put through the firewall. The large rubber grommet that goes through the firewall has two little lines a gray and black. The black is the one that the check valve hooks to this is also the feed line to the vacuum reservoir. This check valve holds vacuum in the HVAC system while the engine is under load producing less vacuum. If the check valve is bad your air flow from your heater/ac will go to you defrost vents when the dial is set to vent. Got the new check valve installed and she's happy. You could blow and suck through the valve both ways. Cut valve apart and found rubber had deteriorated. There is another write up here also that I found later. Hope this helps someone.
