Brake question- special master cylinder reservoir cap - atmospheric vent?


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Vancouver Island, British Columbia
I have been refurbishing the disk brakes on my 83 650sk. I ordered in a kit that had a replacement cap, metal plate(bush in the parts list), diaphragm and screws. To be honest I wasn't really needing a new cap, but I was comparing the old and replacement caps and noticed something. There is a groove cut into the underside of the cap. I assume this groove allows ambient air pressure into the top side of the diaphragm? So what is this called? Atmospheric vent? No mention of it in the Haynes or Chilton manuals, that I can see. What happens to your braking if this vent gets plugged up with crud? The cheap replacement cap only has part of the groove molded.


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I have an '83 with that replacement kit on it. I never scrutinized the underside. The brakes work as well as they ever have. :shrug:
The diaphragm is there to keep the fluid side of the system separated and sealed from atmosphere. I’m sure you know that.
The groove in the cap is to allow air into and onto the diaphragm.
As your pads wear more of the reservoir fluid goes into the system, if you didn’t have a vent it could create a vacuum above the diaphragm, quite how that would effect your brakes I’m not sure. I don’t think it would lead to brake failure but to be on the safe side make sure the vent is there.
Well yes, it is after all called a diaphragm. So I am guessing it will be doing diaphragm type stuff. If the vent gets plugged, then you won't have the ambient air pressure on the top of the diaphragm. Probably works fine if there is plenty of fluid in the reservoir and gets worse as the fluid level drops. I wonder if you would have problems getting the appropriate hydraulic pressure when applying the brake? Just guessing. Any brake experts out there?