Worked on a car that had been sitting unused in a garage 3-4 years. Owner and I got all the major things working except the brakes.

Brakes would stop the car if you pushed the pedal down as hard as you could. But long stopping distances.

Pulled the hose from the booster after it sat overnight and got a satisfying noise of vacuum reserve.

Guessed that either the booster had failed or all 4 calipers were frozen. Replaced one obviously-frozen caliper. No change.

Just in case, pulled the brake-booster port off the intake manifold. It was clogged with a mud-dauber nest. Cleaned that out and the brakes now appear to work as they should.

Best guess: someone left the booster hose off at the manifold long enough for a mud-dauber to build in it. An early repair attempt?

The mud let enough air through to charge the booster a little; but not enough to assist the pedal.
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240 drivers / parts cars - JH, Ohio