Jeffrey, I had the same problem (brake sinks to floor under steady pressure,
but brakes work fine). It was a pitted master cylinder and piston seal.
These things are expensive to buy new so it is worth a hone and rebuild,
however, it is much more complicated than a clutch master cylinder. I
bought a new one ($280 locally!!) and have the rebuilt one as a spare.
We bled the brakes before tearing apart the master cylinder, no effect.
My impression is that air in the lines causes a soft pedal which can be
pumped up to be hard, whereas this leakby problem causes a hard pedal which
sinks under steady pressure.
...John Trindle
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