Older MC use a small vent hole in bowl cap to let air in as level goes down, newer ones are sealed with a diaphragm type gasket. Worth retro-ing a diaphragm type cap seal or not worth the effort? I figure the intent of the diaphragm seal is to keep air [water] from contaminating the system.
If you live in AZ, there's no need. But if you live in CT, with all that moisture in the air, and not driving the car all winter, maybe it would be a good idea. Easiest way to do it would be to adapt a 1960s MC that came with a diaphragm...the Chevy "fruit jar" is one. But if it's a funky MC in an older car that fits under the floor, or something, then it gets difficult.
here's another one, on a 30 year old mopar. Might not be very easy to adapt, but at least the diaphragms do exist in about the right size for old MCs?
All I am going to say is that you don't see the crudded up brake cylinders on rigs with the diaphragms in the caps that you do with older rigs that are vented to the air . Still a guy can probably get away with flushing the brake system out with fresh fluid every spring and call it good. Back when I was teaching High school auto mechanics one of my advisors was the local NAPA dealer and he got the EIS brake rep to make stops at the school and put on brake clinics for the kids and one thing the rep was adamant about is don't buy larger quan***ies of brake fluid than you can use up in a reasonable time as even the container and especially a can of fluid with a fluid pump in it draws moisture into the fluid.
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