Rusty I have to disagree with you.:(
The vent/s in the cap allows the gasket to expand and contract. The system is sealed by the MC gasket. There are bellows of sorts and as the pads / shoes wear, the fluid level goes down and the bellows gets sucked into the top of the MC a bit.
Ever notice that sometimes you have to push the bellows back into the cap when you take the cap off.
The vents vent above the gasket, and the gasket has no holes in it.
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