Your cars motor is one huge air pump. A small portion of that pumping capability is used for powering vacuum assist on the power brakes.

Sounds like the hose to your booster is broken, probably at the check valve. Look at the booster to one side of the master cylinder. There is a hose as big as your finger in diameter. Grab it, it should be rather firmly positioned. Follow it. It should be solid to the back of the intake manifold.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician