I have a problem with my coolant system on a '84 wasserboxer.

Before each trip I check the level, the main tank is full the overflow
tank empty. I can drive for days and weeks with no problems, the gauge
runs just under vertical(halfway). If I get stuck at construction or in
slow moving traffic for more than a few minutes the gauge starts to
rise. If I can get moving again pretty soon there are no ill effects but
if it gets too hot, no amount of driving will get the gauge back down. I
stop and check the coolant when it's cooled down and I'm a quart short
from the main tank.

How can this be. Why can I run for so long at normal temperatures but
the van overheat if it stands for 5 minutes. When it overheats why am I
losing all the coolant and where to, I never see the coolant leaking,
I'm always in traffic. If there's a leak why doesn't it manifest itself
under normal running temperatures? If the pressure is building in the
main system when it gets too hot shouldn't coolant end up in the
expansion tank. If it does, shouldn't it make its way back into the main
tank when it cools?

I have so many questions and no answers.

Thanks everyone.
Melvin