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Update- Last I posted, had the A/C vent switching to defrost default on accel., hills, etc. on 93 E-150 5.8
After checking all vacuum lines, pulled the Vacuum Reservior (Rt. Frt. Fender) Rattle inside Reservoir, found from other posts it has an internal check valve that can break off. I was afraid to get a price from the Stealership for a new Vac. Res.
Got a inline check valve from the parts store, spliced it into the Red Vac. Line, Works like new.
Hope this can possibly help others, as I was going nuts trying to tracking this down!