[Trevor78]

Recirculation mode or Max, will close the cowl vent and the kick panel door. There's a vacuum diaphragm under the cowl door, that's what the bulge is for. Unbolt the door surround and remove, suck on the tube and see what happens. May be all rusty and junk or a cracked hose. Check the condition of the cowl block-off seal while there, can now buy them new, although just a piece of curved sponge rubber.

78 AC Vacuum heater diagram by Ben, on Flickr

Firebird cowl air recirculation vent door vacuum setup by Ben, on Flickr

[i4abuygto]

Thank you for the info on the cowl door and the vacuum diagram - very helpful.

With your attached vacuum diagram, I noticed I need a check valve in the system. I found a diagram for the vacuum routing under the hood and showing the check valve in the system. The 403 I have has an aftermarket intake with Holley 750 carb so I will likely need to pull the vacuum for the AC controller off of that carb.
Most of the stock vacuum system under the hood has been butchered with the intake modification so I will have to figure that out.

I acquired the car with a bad heater core and nothing working in the AC heater or blower system. I had to learn the entire system to fix wiring and ducting. Now I have the system working with the high speed fan working and I just have to figure out the cowl door and I should have the basics done. I still need to rebuild the AC system forward of the firewall. After the dryer, all under the hood components and condenser, for the AC system are missing.
I did acquire an AC wiring harness and AC controller harness that is not all chopped up with good connectors. Looks like I need the AC compressor harness forward of the firewall but I will get that when I decide if I am going stock or vintage air on the system.

Hopefully I can install the dash and interior soon.

I did not mean to hijack this thread but it is all related to the heater box re-assembly and install - thought it better here than a new thread about the same topic.

Thanks all for your help.

[Trevor78]

That's the firewall seal.

[i4abuygto]

That's the firewall seal.

I am missing something in the description - I did not have this when removing the heater box. Is this a removable seal and is it located on the under hood side of the firewall.