Brake pressure build-up, advice needed.

[Steinbru@VNET.IBM.COM]

My recently acquired '87 CGT really likes to go.  Now, occasionally, it
has taken to stopping on it's own!  It seems that pressure builds up in
the brake lines.  All four wheels get braked.  If the car sits, the
pressure goes away after 20min to an couple of hours.  Sometimes the
pedal travel is increased after this.  Friday I relieved the pressure by
loosening the four brake line fitting at the MC, and the pedal relaxed
somewhat.  Pedal travel was normal, and the brakes were not dragging.
What could be causing this?  --Gary