Why is the 911 brake system "open" instead of closed?
 

My '86 911's brake system has a vent, I don't recall this feature on any other cars. Do other cars a vented cap perhaps?

My other german cars have had a vented cap.

A vent (whether a cap or vent like on the 911) is necessary because the volume of the system changes as the brake pads wear. If it were sealed the increasing volume due to wear would create a vacuum in the system.

^^ +1000 This. They're all vented AFAIK - unless they've started doing something funky on those modern vehicles.. Most older caps are pretty obvious - little hole in the top. Some are more discreet - but they've got to equalize the pressure by allowing air in both directions.

Then consider both that brake fluid is hygroscopic ("aborbs moisture") and that the system is vented to atmosphere - and the reason for a regular brake fluid change interval should be clear. The stuff doesn't wear out sitting in your garage - but it does absorb moisture, every day.

My "new" Corvette has two rubber lids on the reservoirs for the brake master cylinder. The lids have accordions built into them that allow the fluid level to vary depending on pad wear and heat expansion of the fluid.

The fluid gets contaminated even in a "sealed" system. I recently flushed the fluid in the Corvette and it seemed like it had never been changed, it was so brown. Therefore I have little faith in the car's master cylinder, caliper flex hoses and the caliper seals. So those are next on the list of things to be fixed/replaced. I'm certain the clutch fluid is equally bad because the clutch pedal feels as mushy as the brakes.......

My 2017 Silverado has a giant reservoir with what appears to be a sizeable enough air void above the max fluid level to allow for fluid fluctuation and expansion.

Kevin - my much modified "GT" 911 now has a racing aftermarket reservoir with the bellows separating the fluid's surface from the vented to atmosphere space in the cylindrical reservoir. It has two outlets (to support dual MCs) but could be used for the normal MC. Better mousetrap for keeping the fluid dry. Kudos to Corvette for doing this even if it costs a bit more.

Hard to say just how much difference this makes. On a race car, how long would you go between brake fluid changes even if you never got a soft pedal? Comforting to have it, though.

FWIW, the cap on my '71 is NOT vented. I am guessing that the overflow acts as a vent and the boosted braked cars are different?

OK, a search of the parts catalog shows the same cap used upto around '83. There is no cap listed for Carreras.

Gordon - the overflow system is a vent - the line you have to pinch off if pressure bleeding. At least for the SCs, with their boosters, same deal. It may be that much newer cars have the vent in the cap (like my race reservoir with the bellows, from which overflow isn't really possible)- the Cup cars I inspect at races often have some cloth held on around the cap with a rubber band, so maybe the vent system on that reservoir is just in the cap and can weep under high G loading.