If ther are no visible leaks, failing m/cyl seals could very well be the
cause.
When was the last time the brake fluid was changed?
Mike.

Could be anything, but a master cylinder failure is unlikely to have
recovered a few minutes later.  Simplest cause might just be that the
fluid is old and damp - then it boiled on you. It could be the pads
fading out, especially if things are worn around the front end, but
that usually needs noticeable heavy braking to cause it.

It _is_ pushing 10 years old and I bet that the only maintenance it has
ever had was new pads. As a bare minimum, I'd want all the rubbers
(seals and hoses) and fluids replaced before I thought about driving it
again. It's a pain to do anything at all to a braking system, but once
you've split the things apart once it's little extra work to do the
whole lot in one go. I'd check the disks too.

Unless the stripped master looks obviously damaged (polished or scored
inside) then I'd be happy enough to begin by replacing its seals,
rather than an exchange cylinder. Obviously this requires sufficient
competence and clean working space to rebuild it, but it's not rocket
science.