I have seen several vans with loose clamps on the plastic lines that go to
the radiator.  Some times the clamps themselves are rusted to the point that
they are tight if you try to turn the screw, but are no longer clamping.  If
this is the problem then it should be just a matter of some new clamps.

The other thing that seems to happen is that the plastic pipes themselves
deteriorate with time.  They are moulded around a piece of metal on the ends
to give them beef when you are tightening the clamps.  Many times the plastic
gives way on the end allowing the metal piece to work its way loose of the
plastic pipe.  You can shove the metal piece back and reclamp but at this
point you might want to start saving up for new plastic lines.

Or (not to leave out the obvious) you could have a leaky radiator (it
happens).

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242