Definately a master cylinder problem. Whether that's related to the master cylinder itself, or low fluid from a leaking brake line, hose, caliper, or wheel cylinder, the master cylinder is where to find out.
If the fluid is empty (just because it happend so dramatically and quickly), which is what it sounds like to me, then you've got a brake fluid leak.
If your brake fluid level is good, your master cylinder is bad.
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