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The military single master cylinder is vented on the top, with a line that goes up to the air cleaner. The dual master cylinder is not vented at all. Does anyone know why?

Dual master cylinders usually have a rubber diaphragm type lid gasket that seals the fluid from the air chamber compartment built into the lid which is usually vented through a very small hole in the lid or gasket edge. Not providing a vent in a master cylinder above the fluid reservoir will result in the master cylinder's piston eventually being unable to pull fluid into the bore.

The original M38 master cylinder vented to the air cleaner in order to get the point-of-vent above the fording water line.

CJ jeeps using the same master cylinder as the M38 have a different cap which simply vents at the level of the master cylinder. They are not suitable for the deep fording for which the M38 was designed. The dual MC similarly is not suited for an amphibious beach landing.

Herm the Overdrive guy sold me a dual master cylinder with a conversion kit. My guess us it came from a CJ5. He tells me that it's not vented, and to his knowledge, they only vented the early jeep master cylinders