Last time this light came on, found a leaking slave cylinder on the rear
axle.  Replaced the slave, flushed the lines with clean fluid, bled the air,
and all was well.  Braking action is normal.  165K miles without much fuss.

Have checked all 4 wheels, no indication of oil staining, has thick shoes &
pads, no scoring.  No stains at master cylinder, brake fluid level is
normal.

Brake pedal is a little soft but it's been that way for 25k miles.  If pedal
is held, it slowly goes to the floor (20 seconds) but it brakes well with no
pulling, noise or pedal feedback.  Am thinking the master cylinder is
leaking internally.  How do I diagnose that?  How difficult / messy for R&R.
$200 at a garage?  TIA, F