The BUTTON?  That releases all the dark matter in the universe...DON"T TOUCH IT!

On my 80 when I had this problem it was a small and I mean tiny vacuum hose that went through the firewall right behind the distributor. Repaired that and I didn't need ice on my feet. Just another thing to check out. 

I'll checkm out. I didn't think vacummm hose.

As mike mentioned, there are three vacuum hoses that come through the firewall near the distributor on his 80 and my 82, one goes to the water valve.  I also had the vacuum hose that controls the water valve break at the rubber grommet, it looked good but when you tugged on it, you could see the break.  I know I broke mine removing and installing the distributor shielding. This hose goes through the firewall to the vacuum "switch"(valve) located behind the glove box and operated by the cable from the heat / cool slider control. You may want to check this "switch" to see if the mechanics of the heat control are engaging it properly to allow vacuum to pass through the hose. If you have a vacuum tester, you may try disconnecting the vacuum hose from the water valve and seeing if you have vacuum.  You can probably test it with a fingertip and feel the suction.  You should have some vacuum unless the heat slider is pushed to the "heat" side.

I posted this on another thread.