Vacuum seal at brake booster?

No one but you can find the source of the leak. I can say that there is a rubber diaphram inside the can.
When it fails there will be a vacuum leak.
Yours can be rebuilt but I think that you can still buy new boosters.
Test the line that runs from the engine to the booster. It is possible it has rotted through or has a hole from road damage.

Thanks doug,

I bypassed the line that runs to the manifold and looked for a vacuum seal directly at the the booster. This is where I do not get a seal. Sounds like a diaphram to me. Is this
common? I'm not sure how to test it.

Yes it is common. The seal will last for 30 years +/-. Some seem to last forever. It's about time.
The diaphram just wears out.

Tom, How about just let the engine idle and remove the hose from the booster and hold your thumb over it. The engine will idle down if you have the leak stopped. How well do your brakes work? I would think the pedal would be hard if the booster was not holding a vaccum. Another test would be to put a hose on the booster and suck on it.......it should hold your tounge to the tube.