That's why I don't use it any more.  Put some in my '81 Jetta to seal a
leaking radiator (this is a while back).  When I installed the new radiator I
flushed the system out really good (I removed the thermostat and everything).
I thought I got most of this stuff out right?  Nope.  The next thing I know I
have black tar coating the inside of my expansion tank.  I thought it was oil
at first but it didn't smell like oil and was sticky.  I have been trying to
get this stuff out of my car's system (way smaller than a Vanagons) for a
while now with no real luck.  Also it seems that Bar's leak doesn't like
rubber.  One of my coolant hoses burst on me while waiting in traffic a few
months ago.  After examining the hose it was coated internally with the bar's
leak and the rubber had gone rotten from the inside, out.  It had the effect
on the rubber that oil has (it breaks down the rubber).
This is another reason why I don't like it.
The only way I can figure to get most of it out is to drain the coolant while
it is hot.  That way most of the stuff is floating in the coolant and can be
drained.  If you wait for the system to cool down it just sticks to the inside
of the coolant system and you will get a little of it but not all.
Ken Wilford
Van-Again
John 3:16