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When the pedal sinks to the floor, it's a master cylinder (m/c) problem. The booster assists the m/c, but doesn't substantially alter its range of travel (only a little, per the effort needed to press on pedal).
You can have a m/c leak without any exterior sign of leakage -- in your case, since you didn't say you were losing fluid, I'd guess it's case no. 2, although I'm also including here possibility no. 1 for other readers:
1) if your brake fluid level in the m/c reservoir is low despite no exterior signs, the rearmost seal on the master cylinder has failed, allowing the vacuum in the brake booster to suck the fluid into it.
2) if the fluid level in the m/c reservoir remains full, then you have a failed seal on the piston inside the m/c. With the piston back, fluid is continuous between the cylinder and the reservoir, but as you press on the pedal, the piston advances (cutting off the passage between cylinder and reservoir) and the suspect seal retains the fluid in the cylinder so that it can be pressurized into the brake lines while also pushing back and resisting the pedal until you can't press the pedal any farther; but if that seal fails, the fluid (in a sense) recycles, flowing back past the seal and going behind the piston and back into the reservoir -- there is no tell-tale sign of any loss of fluid, because the total volume of fluid remains unchanged -- you're just moving it around (while a good seal would "trap" some of it in the cylinder). However, instead of generating increased pressure in the brake lines, the fluid is leaking out of the cylinder and little pressure is built-up to resist the movement of the brake pedal, and you can push the pedal all the way to the floor.
Replace or rebuild the m/c.
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