Tooljunkie said

The old style caps, as in the 70’s and maybe 80’s had a valve in them. Allowing no pressure buildup unless there was a surge or coolant rises enough to close it. The theory was to minimize pressure buildup when not necessary. Helps with lifespan of cooling system.
If i could find the article i would post it here. It may have been in a service manual.

The older rads will expel coolant until they find the happy medium. Will dribble some on extremely hot days.

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