There is a piece in your door lock mechanism that has worn out. I think it's a diaphragm (I could be mistaken, though) -- in any case, it's just a minor something that has to be repaired. You have to expect such things in a 1976 car -- it's certainly nothing serious. The fact that your doors unlock shows that you don't have a system-wide vacuum problem, which is much more difficult to solve and repair.
I had the same problem as yours, and had it fixed at my local dealer, for a small charge -- I think it was an hour or less of labor, and a pittance for the part.