98 f150 4X4 supercab lariat w/7700lb auto 5.4. When I hit the gas hard w/ac on the air will start going out the defroster vent until you let off the throttle. The air will then return to normal going out the dash vents. I found two vacuum hoses broken behind the battery tray and fixed em. The little vacuum tank tests ok. I'm thinking maybe a bad check valve but not sure where to look. Any help greatly appreciated.
98 F150 4X4 Lariat 5.4
93 full size Bronco 302
68 Corvette roadster 427 3X2

 

the common areas that leak are the hoses inside the fender well as you said, and on the firewall, between center and glove box--- thats where the vacuum hose goes thru the wall. look for a small TEE fitting, and 2-3 hoses. TEE can crack, hose get hard, swell, etc.

 

OK, finally figured this out. Got some great info from looking at posts by members "Aftrmidnite" and "steve(ill)". Turns out the little one-way check valve behind the battery box next to the firewall was bad. Its a three way T-fitting half blue and half white. It should only pass air in one direction (toward the blue end). If you can blow into the blue end towards the white the part is defective. The Ford replacement part according to "Aftrmidnite" is XL3Z-19C828-AA. I found an after-market replacement part in the "HELP!" section of my local AUTO ZONE store. Part #47150. The vacuum end tube is a tiny bit bigger around than the original but with a little WD-40 it slipped right in. Problem solved!!! Thanks everybody.


Previous Ford trucks that I wish I still owned...
1959 F100 stepside 260 V8 auto
1966 F250 Camper Special 352 V8 auto
1972 "Ford" Courier w/CJ-2 Jeep 4X4 conversion kit 1800cc IL4 cyl 4 speed
1978 F350 Ranger Trailer Special super cab 460 V-8 auto