I am pretty sure you have a bad brake cylinder.  Crawl under the bus and
take a peak at the inside of the wheels.  If you have a leaky wheel
cylinder there will be road dust stuck to damp oil. It sort of looks like
dark wet dirt. This should be obvious and may cover the whole inside of
the wheel.  If you run your finger through it you'll feel the oil.

One locking up wheel (and suddendly) is the symptom that told me that was
the problem. I had a Bug that had that once. If it sat for a few days,
the first time i went to stop it would skid one wheel.  Then it would be
fine for a while.

Good luck
M

Matthew Pollard          "Racing with the wind and flirting with death
Dept. Of Chemistry        So have a cup of coffee and catch your breath"
University of Idaho
www.uidaho.edu/~poll7356