Apparently there is a difference between radiators and caps depending on if there is an overflow tank or not. In the cars that I have been familiar with there is a metal tube in the radiator neck and it has been connected to a rubber hose that dumps to the ground. Is there a different setup on cars that had a tank? Some suppliers sell a different cap depending on which type system is in use and I'm having trouble thinking what the difference might be.

Buz;

Without a tank, the radcap is a pressure type cap which keeps the system closed (from the overflow)...when pressure rating of cap is exceeded (called "overheating"), seat opens to tube which dumps out on the ground.

With a tank, the radcap is strictly a hard seal (and totally different style of cap), and the tube at the rad filler is constantly open to the bottom of overflow tank...allowing thermal expansion into the tank, and sucking it back into rad when cooling. System pressure is regulated by a pressure cap on expansion tank. Benefit: System self-burps, and no coolant loss due to thermal expansion. If overheating occurs, tank first fills, then overflows tank as tankcap seal is forced from sealing surface by pressure.

Old style (without tank) can be easily upgraded by mounting and plumbing in overflow tank (with pressure cap), then disabling pressure release system of radcap (but still letting it seal rad)...I've done this by lifting sealing ring off seal with a wire partially around sealing surface.

See also: http://sw-em.com/service%20notes.htm#Cooling%20System

Cheers