Does a radiator cap have a rubber seal on the pressure release disk and the top of the neck?

Yes and no. The better brands will have a neoprene gasket at both places. The gasket at the top of the neck seals with the flange to provide a good vacuum seal when cooling down to draw coolant back from the recovery tank.

Anymore, I believe 95% of the caps made will have a gasket at both locations, since autos are outfitted with coolant recovery. The older vehicles without recovery do not have a gasket at the top. Its cheaper to make one cap for all than to make two caps each application.

If you use the newer type of rad cap designed for the recovery system on an older system that does not have a recovery bottle, when the engine cools and pulls a vaccuum it will pull air into the cooling system, as it has no coolant to pull in.
Yes, most caps you buy at autoparts stores now will be the newer type, only because there are no vehicles anymore that dump coolant onto the street.
All original restoration guys, order their caps from repro places that still stock or knock off the original caps. The older cars didn't have a recovery system.

I think this may have answered my question. It still looks like the cap should have a seal for the top of the neck. Otherwise it is metal against metal. A cap has to be special with a second check valve to let water back in under vacuum.

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[ 03-10-2005, 08:15 PM: Message edited by: Gary L ]