only when car is in max A/C mode does the air coming out the vents shift to the defrost vents when giving some gas. it works fine when in norm A/C mode. I know it's a vacuum leak somewhere. which vacuum hose controls the air flow doors when in max A/C mode? I thought it was kind of strange that the doors work fine in norm A/C but not in max A/C. any help appreciated.

 

In MAX AC mode there is more tax on the AC system. There is a flow control valve on the heater hoses. When in MAX AC mode, vacuum is applied to this valve, closing of circulation of hot coolant to the heater core. This ensures that there is not heat buildup that could otherwise make the AC less efficient.

It is possible this circuit could be leaking, or it could be that the extra demand is enough to make another leak elsewhere more obvious.

 

what make/model vehicle? in my 97 v8 explorer, there is a small black plastic globe resovoir sitting just inside the front passenger side of the bumper. there are small black plastic vacuum lines coming from them going to the driver side of the engine. there may be a crack in them somewhere along the way...

 

it's a 97 Explorer 4.0
I will check that out, thanks.