[malc]

I´ve been using a 16lb cap until recently when I thought I´d try a 17lber.
No good reason, just to see what effects it has.
Drove the car into town to the upholsterer, about ten minutes, water temp reached 180ºF.
After awhile I came out to the car and an old guy was looking at it,
wanting to know what it was.
Showed him the interior and when I opened the hood I immediately noticed the overflow tank was right down to empty.
The heated engine had sucked water in rather than it expand out when hot.

Is this something I should expect ?

[LATECH]

Yes. The cap should also allow a warmed up engine to draw in coolant as the thermostat opens, when pressure starts to build from the heated water it closes the valve in the cap and holds pressure.
a warming engine expands any air in the system, when the thermostat opens, the engine gets a big "gulp" of cold coolant, which will cause any trapped air to contract and pull a slight vacuum on the entire system.The cap seals the system to the overflow tank and when that cold gulp occurs the vacuum it creates pulls coolant from the overflow uilds as the hose is submerged in the overflow tank.Once it does, pressure builds and causes the cap to seal the valve to the overflow, as now all the water in the engine is warming. Its a vicious cycle. LOL