In a liquid cooling system that has no *non-condensible* gas (i.e. air), the pressure will rise due to thermal expansion of the liquid, until it gets to the pressure relief value of the cap, at which pressure a small amount of the expanding liquid will go into the overflow bottle.
Upon cooling, the liquid volume shrinks, and the pressure decreases until it is lower than that in the bottle, and liquid is sucked back into the system. If the bottle is pressurized, then the pressure in the system will only go down to the pressure of the bottle. This may be what is being described in Dave's system.
Dave, do you have a *pressurized* overflow bottle?
Bill Schertz
KIS Cruiser # 4045
Bill, yes, I have a prssurized overflow bottle (that's a lot easier way of saying it than my other posts :-)
Will try taking the cap off that and running the system, that should sort everything out. Thanks!!