Why no Vacuum at Heater Valve?


Active Member
Jul 17, 2020
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Cleveland, OH
1970 300 Hurst with 440 auto. Saved from junkyard. Dash and firewall was rotted, but I got almost everything to work. When I bought it over a decade ago, heater/ac buttons were stuck, so I kept pushing/lubing them until they operated smoothly again. In the last few months, I haven't heard the "pssshhhhh" sound anymore when I go from "off" to push in the "heat" button, although fan blows hot air. My A/C has never worked but I noticed it blows hot air too. Around the same time, I disconnected and plugged the vacuum hoses to the fan door since it was rotted and I would rather have it open to interior all the time anyhow. These aren't connected in anyway to the heater valve, are they? None of the vent doors work due to rot, so heat comes mostly out of the A/C vents, with a little at defrost and at floor.

I traced and inspected heater valve vacuum hose all the way back to under the heater/ac controls but can't see anything due to the huge 8 track in the way. There doesn't seem to be any leaks but not sure. I can't hear air escaping and I'm pulling 18-20 of vacuum at carb.

So what part went bad? I'm thinking of just installing a manual turn valve instead of the heater control vacuum valve. I know I'll have to go under the hood and close the manual valve every time I want to use A/C (once I fix the A/C), but I don't want to take the whole dash off just to fix this. Thanks for your help.
You probably have a bad vacuum switch in the heater/AC control. The other possibility is that the vacuum port from the manifold connection is plugged. The control draws vacuum from a small branch fitting below the connection for the power brake booster. Try removing the vacuum line from this fitting to see if you have vacuum. These small fittings sometimes get clogged with small bits of rubber from decayed hoses. If you have vacuum there, you probably have a bad vacuum switch which will need to be replaced. There is a vacuum plug on the back of the heater control, check to see if there is vacuum there before trying to remove the heater control for servicing.

I know I'll have to go under the hood and close the manual valve every time I want to use A/C (once I fix the A/C), but I don't want to take the whole dash off just to fix this.
You don't have to open the valve unless you need heat.

Chances are your problem is the vacuum switch. Common issue... They kind of come apart when the glue dries out. Here's an article on repairing it.

You should be able to get to it by pulling the radio out... which isn't a big deal. I take the ashtray out to make it easier.
The 69-70 300 models were equipped with a time delay switch (as @Big John suggested) located behind the glove box next to the fresh air recirculating door actuator. If found unplugged or vac line(s) missing can cause no A/C actuator operation. Also degraded foam seals in the heater box will cause improper air flow as you stated. In the case of no heater temp blend Check for kinked or broken vac line(s) in either the supply vac hose to the vac controls as seem in the diagram below. Removing your radio will greatly aid in the ease of repair and diagnoses DO NOT attempt to rebuild the vac control switch, there are too many out there in NOS condition and will give you fool proof service for many years. Any Questions PM me.