Replace the cap.  Good thing to do every couple of years anyway.  Sounds
like the valve is stuck and won't let the coolant back in.  Radiator cap
is just basically a two way valve. Releases pressure when it hits the
preset point (usually 15-17psi) ( Pressurizing the system raises the
boiling point of the coolant.  I can't remember the formula.)  Then when
the bus cools off, a vacuum is created and the valves lets coolant back
in.
--
                             -Jim DiGennaro
     '84 Green Westfalia 'The Turtle Van',  '90 Jetta (Brides car)
                  '92 Scamp Trailer 'The Turtle's Egg'
                              ICQ# 5534528
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