I have this low mile 4.6 1997 F150. It is doing the same thing my 03 4.6 Expedition is doing. On a slight incline, when the vacuum drops, the vents close. Only floor and defrost. Once you back out of it, couple of seconds later the vent reopen.

I have tried, new FOMOCO reservoirs, and if 1 is good then I put two in. Still acting up. What am I missing? I am stumped

 

Maybe missing a check valve on the vacuum line somewhere.?

 

Thats an idea--let me check for check valves