Need some guidance on where to start looking for a vacuum problem. This is my starting point: 2003 Expy, standard climate system (NO EATC) A/C will work fine in "Regular" a/c mode (at least getting as cool as it can in town with outside air in 103 Texas heat). "MAX (recirc) mode works only at idle. As soon as I step on the gas and the RPM start climbing, the dash vents slowly shut down and I can feel the air shift to the DEFROST vents. Back to idle (at a stop light) and it shifts right back to dash vents. Since it is RPM relative and the vents shift to the "default" position at DEFROST, I am obviously thinking a vacuum problem somewhere between the a/c control unit and the Recirc Door Vacuum Motor? Don't want to go tearing into the dash and the a/c control unit before I rule out a vac leak under the hood or under the dash. Can't seem to find a definitive vacuum "map" to trace the vac line back thru the firewall to its source. Where would you start the troubleshoot?