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

Looks like you have it figured out pretty well. It’s not supposed to hiss except maybe briefly when the switch is activated. Wherever the hissing is coming from need to be replaced

Bob is right it is not supposed to have a vacuum leak on that shaft. I see you are from the northeast like me and use your heat at times? I am not sure if you are aware, but you are one of the lucky first gen owners whose car comes with a hot water shutoff valve, 69 birds with a/c only. The rest of us first gens did not rate even though the more expensive models in the lineup had been using them even years prior. The reason it's so great and why all cars after about 71 used them is it keeps the cabin cooler in the summer because even though the blend doors close off the hot air from blowing into the cabin you still end up with the core and the whole case under the dash at full engine coolant temp which of course is just like a convection oven under the dash. The reason I went through that lengthy explanation is because if it were me, I would want to keep the hot water valve in the system. The problem with the control valve leaking is the gasket or O-ring is dried up or broken and finding a good used one as you found out will be a crap shoot and I don't think they are available in the aftermarket, at least a direct fit. The simplest thing you could try is to put a drop or two of oil at the top of that shaft and let it soak in good and if the seal is not too bad it may swell and seal. It's a long shot but easy to try so why not. The second thing to try if you are handy and really want that controller to work is to un peen the end of the shaft of the worst of the two you have and see if you can replace the seal inside or find an auto hvac co. that would be willing to do it for you. To me the easiest solution if you're not putting the a/c back into the car is to plug the vacuum line to that valve and just manually plug and unplug vacuum to the hot water valve under the hood when you want heat which would mean If you used the car in the fall just leaving it in the heat position and in the summer in the off position. Of course, if you don't care about the hot water valve at all you can just eliminate it and deal with the extra heat in the summer like the rest of us peons have to.

Thanks for the replies, but I am on my second part and they both do exactly the same thing. the guy I bought the part from said there are no seals or gaskets inside that part. And the valve works proper when it's supposed to send vacuum to the heater valve in the engine. there is no sound at all, no leaking. it's when you move it to cold, it dumps the vacuum (or more accurately, is probably sucking air in) through the connection point making a noise. I can't imagine two parts are bad. what is really weird is that you'd think this would be a parts bin part and multiple GM's would use it, well they do, but not this exact design. it's unique to 69 Firebirds. I can't tell you how many parts are unique to this car. I will try the oil thing, but something just seems weird here. you can see in the one diagram that there is a part on the travel of the valve where it's not sending out vacuum to the heater valve and it's quiet holding pressure inside (I've tested this with a vacuum pump). but that area is not where the pin goes on cold, it goes in the opposite direction and dumps the air. I know the sound can't be right, but two bad parts? just seems odd

I need to call foul here. I think some idiot (that being me) had the vacuum lines switched in the engine bay. when I switch them on a vacuum pump on the part, it holds an internal vacuum on cold (no noise), and send out vacuum on hot on the other port. hey, we all make mistakes

Ya, I was trying to figure out how to nicely tell you that a vacuum controller does not work like that creating vacuum leaks. It either shuts it off or redirects it, one or the other. Glad you ended up getting there.