have the whole dash apart trying to chase down an annoying hissing sound when the temp control is on cold on my 69 Firebird with A/C (which is currently not in the car). What I've found is the water control valve is basically an on/off switch. While the vacuum switch rotates quite a bit when the temp slider moves from cold to hot, once it's about 10% on its way to cold, the valve sends out vacuum to the heater valve in the engine, so it's just either sending out vacuum or it's not. The cable that is hooked up to the temp slider likely controls air mixing (guessing).

The issue is when the temp slider is on cold, you hear a hissing sound. I have confirmed that the lines coming in seem tight and no leaks. the hissing is coming from the valve itself. I bought another valve to confirm and it too dumps the engine vacuum when on cold out of the center of the valve (at the pin). The diagrams attached might help, which I did consulting with someone to show them what it's doing. When it's on cold, air is dumped out of the valve and it makes noise. I guess I'm wondering if this sounds correct and that it's just that in my car, you can hear it because it's excessively loud and, normally, it's so quiet that you can't hear it behind the dash. I'm really at my wits end trying to figure out why it's hissing, so any help would be greatly appreciated