it is easy to fill a straight verticle pipe, closed at the bottom, open on
top, with water. the pipe can be filled from the top by just pouring the
water in, or from the bottom by pumping through an inlet. either way, all
the air gets buoyed up and out.

so why is there so much black magic about refilling and purging the air
from vanagon cooling systems?

a clue might be that when a cup of coffee is turned upside down the coffee
spills out. let's say the area of the free liquid surface is 30 sq in.
then the force on the surface exerted by the atmosphere is
15 psi X 30 sq in = 450 lb, which is much greater than the weight of the
coffee. therefore, the atmosphere should easily hold the coffee in the cup
when it is turned upside down. essentially, the atmosphere is pushing from
both directions, squeezing the cup and coffee together. but the coffee
really does spill out, despite all the squeezing. the reason is that the
lighter fluid (air) cannot support the heavier fluid (coffee) because of a
fast growing instability. similarly, it is not possible for the free
surface of coolant in a pipe to push an air bubble down - the coolant will
just fall through the air bubble, which will then end up on top. whence,
if there is a local maximum in height, a place for an air  bubble to
reside, it is non-trivial getting all the air out. therefore, it is
necessary to run the engine at a high rpm, or use some other pumping
method, to circulate air bubbles up to an elevated bleeder.

if the van's cooling system was like a straight pipe, all that would be
necessary would be to fill the system from its highest point, and all the
air would be buoyed up and out.

dlk