I learn something this weekend! (I know, basic stuff, I should know
better!!!!)

The expansion tank will suck coolant from the refill thank until the
expansion tank if full, only then the refill will have some coolant. If you
keep the expansion tank at 3/4 (It's ok to run the van like that, I've been
running my van like that for the pass year!!! At � with the refill
empty!!!) the refill tank will always be empty, if you put just a bit of
coolant in the refill tank it will go right away to the expansion tank
until this one is full. When the cooling system is functioning properly,
the "coolant Expansion difference" is about 1 inch between hot and cold.

I just fill my exp. Tank full this weekend and left about 1/4 in the refill
tank.

If you leave the expansion tank at 3/4 and the refill empty there is 2
disadvantages:
-You need to remove the rear hatch to check coolant level.
-If you have a sudden coolant failure, the exp. tank will drop faster but
the result will be similar as the coolant level sender is in the
exp.tank!!! the only difference is that (if refill tank is always at �) you
will not know right away that you have a problem.

In case of a hose failure (coolant going fast!!!!) it would be better to
have the coolant sender in the refill tank!!!!!!!! More time to stop the
van� (Coolant light on, "full" expansion tank left before major failure)

Regards, Ben
http://www3.sympatico.ca/huotx/engine1.htm