In that you're from the frozen north ... where chemicals are put on the
roads in winter, I'm betting that a steel brake line rusted then blew
under pressure.  A mechanic here in Houlton, Maine says he's replaced
more steel brake lines in the last 3 years than in all the 15 years
prior.  They're using a different road treatment nowadays and thorough
brake line inspection is routine.

DOT4 brake fluid has a higher boiling point than DOT3 and is a better
choice.  The two are compatible.

Mike