Thanks to all who answered!!!

I now have a much better idea of what the problem is and how to fix it.  I am
almost 100% sure it's a crack in the expansion tank, since the coolant level
sensor is relatively new (1yr).  But before I start working on it, I have a
couple more questions:

1.After I change out the expansion tank, how do I refill the system with
coolant and purge it of air?  I had a mechanic flush the system just over a
month ago and never done it myself (but would like to learn).
2.Do you have to use alot of force when pulling the level sensor out with the
pliers or not?

Thanks so much,

Jorge
1990 Westy Vanagon Multivan