I have a 2000 Chevy Express with 122,000 miles. It has the high top Mark III conversion package. I recently had my local mechanic change the AC compressor and dryer. The front ac woks very well, but the rear ac is blowwing hot air. We pulled the access panels and found the rear blower and found the water hoses to be hot, ie the heater was running. Is ther a valve or vacum switch malfunctioning? The control switch for the rear ac is only a speed control.
I too have the same issue. Jacked up the vehicle and located what appears to be a refrigerant flow valve about midway undercarriage on the passenger side. I remember traveling through a large body of water when it failed shortly thereafter. I believe that the flow valve isn't opening up to allow refrigerant to reach the rear evaporator.