John Rodgers :  we seem to agree, except for what
happens under NORMAL operating conditions - IMHO the
expansion (overflow) tank is inactive under NORMAL
operating conditions and is only used when the system
begins to over-heat and the cap "lets go" above its
specified setting.

I have read today's posts on this topic.  What I meant to
say was that  AFAIK, the Coolant Reservoir cap pressure
relief valve is set higher! than the "normal" operating temp
of the engine - when warmed up.  My experience is that I
can drive around all day (even with external temps at 44
deg. C) and the coolant reservoir cap will NOT allow the
coolant to flow into the Overflow tank - as long as the van
keeps moving!  And (just for the record) when the engine
cools down it is the pressure that drops and no coolant is
sucked from my van's overflow tank - under NORMAL
operations.

When going up a looooong hill (in 3rd gear), or stuck in
traffic, on a HOT day then YES, the fan comes on and if
that does not cool things down, then the coolant reservoir
cap "lets go" and some of the pressure is relived from the
system and, upon cool down, then, yes, coolant is
sucked back into the Coolant Reservoir tank from the
overflow tank via the coolant res. cap.  This "sucking
back" may happen while I drive - as the engine temp
returns to "normal".

So I hope that we agree.  Malcolm