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Your description is as correct as this former design engineer can surmise....Somebody please correct me if I am wrong but I don't understand how these vent caps are acceptable to use on the master cylinder. It is my understanding that they have a small disk that is held closed on the seat by a small, low tension spring. As the gear oil in the axle, transmission or transfer case, that they where designed for, heats and expands the pressure builds and moves the disk off the seat and relieves the pressure. Isn't this just backwards from the needs of the master cylinder. Lets say the the brake system has no leaks to deal with and just normal wear of the brake shoes is involved. As the shoes wear the wheel cylinder pistons will travel outward further and require more brake fluid to fill the space in the wheel cylinder. As more fluid is required by the wheel cylinders the level of the brake fluid in the master cylinder will decrease. If no allowance is made to fill the void left by the lower fluid level in the master cylinder a vacuum will form and brake fluid will cease recharging the master cylinder piston for the next application. So if we install a valve that is intended to relieve pressure, not vacuum, isn't this just setting up for a brake problem.
Use of these vents on a Master Cylinder is a Bass-Ackwards application and will more than likely cause more problems than curing any at all.
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