[Rovernet] Brakes locked 3500S only when car is on - Booster?

[Rob Sayre]

Good morning:

When I start my 1970 3500s the brakes lock and the pedal is hard as if applied.

If the vacuum line from the intake to the booster is disconnected everything frees up.  All lines from the intake and between the booster and the master cylinder with the white plastic valve assembly are intact with no leaks.  The little valve under the cap with the spring is not leaking any air and is pulled closed by vacuum.

I took the white plastic assembly apart and the diaphragm  has no visible defect.

The car sat for several years.

Any help on how to pinpoint and fix the problem would be appreciated.

Thanks,

rob

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