Hello,
I am having a problem with the heater control on my car. I have had a slight "hissing" sound in the area of the control pushbuttons for some time and an issue with the vents switching modes on their own when the engine is under load (passing, climbing long hill, etc.) I have checked and replaced the main feed line from the intake manifold and put in a new check valve. I have also swapped out the control head itself (twice). No change. I still hear the noise and when I shut down I can hear the vacuum slowly escape. I did get the replacement controls from a salvage yard, but it's hard for me to believe that three of them would all be bad. The problem seems to be isolated to the "recirc" button. When I push that one in, the hissing seems to go away. Where should I look next? Thank you for your help.