I was typing this as Patcon responed?.  One more thing to consider is the check valve between the magnet valves and the vacuum canister.  45 years is a lot to ask from a plastic part and they do go bad.   With your heat on and the engine running,  pull the vacuum line on the back side of the check valve and put your finger on the hose, you should feel some suction.  If you have suction there , put the hose back on and pull the vacuum hose on the other side of it.  You should feel suction from the valve there also.  If not,  the check valve is bad. Bay and Amazon sell them, 1/8" is the nipple size.

 

Edited March 8, 20205 yr by gwri8
Picture added

I somehow figured a way to test mine by blowing air into it and listening for leaks. It did not leak so I moved on to the valve. It leaked air!

If you have not done it already, you should hookup the heater hoses, then pressure test the system. You should pressure test the system just because its a good idea to find other issues as well but it will certainly help find leaks.