If the shuttle that shuts off one end of the brake system when it leaks is your problem, the shutttle can be returned to center.
What you need to do is fix the problem that caused it (bad line, hose, wheel cylinder, etc). Then top off the master cylinder and create a leak in the "good' end of the system by opening a bleed valve on a caliper or wheel cylinder. Then slowly and carefully depress the brake pedal. When the shuttle centers, you'll feel the pedal harden up, then go down ever so slightly. Stop pressing on the pedal and close the bleeder. The reason to be slow and careful is that if you don't do that, you'll just move the shuttle through center to the opposite position, shutting off the other end of the system. To finish up, you'll need to bleed the brakes. (Note that you couldn't bleed them previously because there was no way to push fluid to the end of the system you just fixed).
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