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Over the years, I've experienced that on two old cars I've owned -- both times it was a prelude to Master Cylinder(!) failure. It's an intermittant failure of the internal seals on the piston. The first time, I ignored it (because it was only an occasional thing) until it suddenly got much worse; the second time I didn't take a chance.

Don't take a chance on your daughter's well-being.

It won't show up as a loss of fluid (as seen in the reservoir). The internal seals that "capture" a sample of fluid in the cylinder are compromised, so instead of only forcing the fluid toward the calipers when you step on the pedal, thus building pressure, some fluid leaks back to the reservoir (so the reservoir level doesn't change) and lessens the pressure buildup. Thus, "captured" fluid escapes and its volume diminishes, and the pedal is able to "sink" farther than normal.

You need to replace the master cylinder.