I don't think you can pinch off the flexible lines without ruining them. They are not just rubber, they have spiralled steel in them.

You could be suffering an internal leak in the master cylinder. Replacement or rebuilding is the only option.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.