Is the coolant coming out of the expansion tank under pressure or is it just
coming out when you start up the van?  If it is doing the latter then it
could just be you have a bad pressure cap.  I have a new one for $6 if this
would help you.
Also if you have a flow problem with a properly working pressure cap, and
your waterpump is working properly, then it is probably your radiator (Bar's
leak or age has clogged it up).  I have new ones for $175 if you need one of
these too.
Let me know if I can be of any more help to you.
The other two things you might want to check for are, stuck thermostat, and
scoring of the case behind the water pump (caused by a previously
catastrophically failed pump).
Ken Wilford
Van-Again
John 3:16