A hole who invented our A/c system ? I replaced 2 check valves and the transfercase vent tube check valve. A/c worked for a day or two then bam , wacky vents. No wacky vents with the overdrive off though.

 

Check the vacuum reservoir under the cowl. (where the wiper mechanism hides.) Make sure it isn't cracked. Verify that you don't have any vacuum leaks in the dash either. Another option would be to install an additional vacuum reservoir.....

 

Can you elaborate on the vaccum reservoir under the cowl? Also I'm guessing installing an additional reservoir would mean putting a new one in line somewhere?