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Why would the master cylinder need to be vented to air filter. The brake system is a sealed system.

Same as the gas tank. If you intend to take fluid out of a reservoir then you must let air in or she will stop flowing because the vacuum that would be created prevents free flow of anything out of the reservoir. They both have pumps though the gas is engine driven and the brake is foot operated they still have pumps and the pumps must draw in fluid from the reservoir.

Now if you just don't want to run a line to the air cleaner then that's not a problem. Same with the gas tank. They both must be vented to the atmosphere but they only need to be plumbed to the air filter if you intend to submerge the jeep or if you want her to be original.

That explanation raises another curiosity, Why would the master cylinder I ordered, (which I think would fit other jeeps) has a cap that would not allow atmospheric venting. I can understand the fuel tank needing a vent. The fuel pump would pull a vacuum on the tank if not vented and gas flow would stop.

I plan on tubing my components up as it (the jeep) was built though. But the M.C. venting doesn't make sense to me. There isn't a pump continully sucking fluid from the reservoir in my opinion. And usually there is a small blanket of air in the reservoir to aid in fluid flow. If the brake system is in good working order and adjusted right the fluid flow is very very small amount.

My wife just tells me to shut up when she gets tired of my questions.

Ken,

If your MC does not hve the vented cap, you can get one from MWM for about $10... I did and have vented it all...

As to your question ????

Even the civvy caps without the 1/8" Pipe thread hole have a small drilled passage with a baffle inside the lid for venting.

Everytime you let your brake pedal return from the applied position it draws fluid from the reservoir.

Another point to keep in mind is thermal expansion which is also accomadated by the vent.

Thanks Wes, My question has been satisfied. By inspection of the cap I found the small vent hole (was difficult to see) drilled in the top just below the hex part the wrench fits on.

And I always thought the MC was sealed.

Now I know.

Ken