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vented brake cylinders

vented brake cylinders

vented brake cylinders

Generically, at least on pre 1990 vehicles US cars master cylinders have a bellows under the cap to compensate for fluid level changes.
Seems like European cars have caps vented to the atmosphere.
Motorcycles use bellows, probably to prevent leakage when bikes tip over.

Does anyone know of applications that are exceptions to this "rule?"

RE: vented brake cylinders

I thought the bellows was the 'modern' way to do it, because it keeps atmospheric moisture from affecting the brake fluid, or at least retards the process.

One way or another, you do need to allow _something_, be it air or bellows, to follow the reservoir's liquid surface down as the linings wear and the wheel cylinders extend to take up the slack.

Mike Halloran
Pembroke Pines, FL, USA

RE: vented brake cylinders

The move away from the bellows coincided with the adoptation of plastic reservoirs on master cylinders as opposed to a one piece cast master cylinder. With the plastic resevoirs, the configuration was not conducive to having a big flat bellows seal and clamp on lid. So, we went to screw on caps with a flat rubber seal with a slit (known as the "burp valve"). Unfortunately, this configuration is somewhat prone to permitting ambient moisture and even engine compartment hydrocarbon vapors from being breathed into the brake fluid reservoir.

RE: vented brake cylinders

My old Subaru had a plastic reservoir with a bellows in the screw-on cap.

Do the vented to atmosphere cars also use silicone fluid? Less hydroscopic than petroleum based fluids.

RE: vented brake cylinders

Not aware of anyone using silcone (DOT 5) other than the U.S. Army. Harley used it for a while, then dropped it.

RE: vented brake cylinders

"Do the vented to atmosphere cars also use silicone fluid? Less hydroscopic than petroleum based fluids."

Volvos with which I'm familiar (240/740/940/850) between 1975 and 1996 have Vented cap screw on and Dot 4. Yes they have plastic reservoirs. Girling and ATE brake parts.

1980 Yamaha 850 motorcycle - no vent, rectangular plastic translucent reservoir, 2 screws secure cap, Dot something fluid.

Here is a round screw cap with bellows, but it is for Motorcycle, thus has spill/tip risk.

This 1940 Chevy MC appears to have a vented cap, but I'm guessing polymer technology of the pre-war era might have made a molded bellows too high tech for production.

I was also thinking the ventless design was "better" in regards moisture absorption.
I guess I would have expected DOT might have stuck their nose into "requirements" for US cars by the 60s, driving the bellows design, since a vented cap would seem to be the economical choice. But then the imports would have had to follow suit.

Even (some) of the M-cyls mounted under the floor, and thus subjected to splash and submersion seem to lack bellows, suggesting a vent is in action.

RE: vented brake cylinders

I have a '52 Ford truck with the M/C under the floor, very much like the Stude's (I'd bet they are both Bendix). The cap is actually more sophisticated than it looks in the diagram, having some kind of plate that "jiggles" to allow equalization and very small vent holes that prevent water getting in or fluid getting out, to a point.

On motorcycles (at least some), even when upright the fluid level is above the gasket line due to the angle of the handle grip. The recharge port is on the uphill side (toward center of the bike). So it isn't purely a "tip-over" consideration.

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