master cylinder vent (why or why not)

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ewgoforth

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May 23, 2010
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Hello,

The Norton Lockheed master cylinder has a tiny vent drilled in the cap and rubber inner cover to seal the reservoir. I assume this allows air to flow into the reservoir to keep a vacuum lock from developing in the reservoir, similar to the vent in a gas tank cap, and keeps brake fluid inside the reservoir and keeps moisture from the air from mixing with it.

I've never noticed any such vent arrangement in any other master cylinders I've ever worked on. How do other master cylinder reservoirs handle this? Is a vacuum lock in master cylinder just not a real problem?

-Eric
 
Greetings,
Most other bikes (read Japanese) have rectangular flat covers held down with screws. If you remove the cover and flip it over you will in fact see a small channel through which air may pass to the top of the rubber diaphram. Its not like your venting a gas tank, but as the brake pads wear the fluid level in the master cylinder does drop slightly. Theoretically speaking you should never need to top off your brake fluid, and of course as a matter of regular maintenence you are flushing and replacing the brake fluid in the system every two years, just like the rest of us :roll:

GB
 
Likely the most vital reason is in summer time the vapor pressure could build to apply some brake drag. Lever could easy over come any slight vacuum and vacuum would only help retract piston and help keep it drier from condensation, that collects and sinks to rust lowest parts in the system. Fluid expands in sun and usage so tends to push excess back into reservoir which would tend to push up rubber cover which pushes on air above needing a vent to prevent pressure back on calipers.
 
The LF Harris replacement master cylinder on my 76' Triumph has a hole in the cap too. On a warm day you might even see a trace of moisture around that hole. We have all seen I'm sure that fluid will get past the rubber diaphram and sit on the top of it.


Tim_S
 
geo46er said:
Greetings,
Most other bikes (read Japanese) have rectangular flat covers held down with screws. If you remove the cover and flip it over you will in fact see a small channel through which air may pass to the top of the rubber diaphram. Its not like your venting a gas tank, but as the brake pads wear the fluid level in the master cylinder does drop slightly. Theoretically speaking you should never need to top off your brake fluid, and of course as a matter of regular maintenence you are flushing and replacing the brake fluid in the system every two years, just like the rest of us :roll:

GB
Click to expand...
That IS the reason
 
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