There are two kinds of coolant "recovery" systems (besides no recovery, where excess expanded coolant is just vented to the ground through an overflow tube that comes out of the side of the rad filler neck. In that case, the rad headed tank ends up with ar at the top, where no more coolant gets expelled, just air.

To have no air in the system and maximum amount of coolant circulating, coolant recovery systems were devised. The original ones were where expanded coolant is forced out into a recovery tank and when the cat=r cools, it's sucked back into the rad. After a few cycles, there is no more air in the cooling system. You can add coolant, if necessary, to the recovery tank when it gets below the minimum level. The recovery tank is not pressurized. That's what I have in my '85 Camaro. The other system is like what I have in my '92 740, where the rad has no filler cap, but a high pressure hose from the rad goes to the plastic expansion http://homepages.donobi.net/sufuelpumps/Other_Subjects/Cooling_and_radiator/EXPANSION_TANKS_COOLANT_RECOVERY_SYSTEMS_AND.pdftank, which has a pressure cap. Basically, this system also keeps the circulating coolant free of air because the air stays in the expansion tank. The down side is that you can't open the expansion tank cap to add coolant until the engine has cooled, the same as not being able to remove the rad cap on a system without any overflow tank.

Here is a more long-winded explanation.

http://homepages.donobi.net/sufuelpumps/Other_Subjects/Cooling_and_radiator/EXPANSION_TANKS_COOLANT_RECOVERY_SYSTEMS_AND.pdf


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1992 745, >500k km