SOHC.co.uk Forums > CB500/550

master cylinder diaphragm

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deltarider:
Just serviced the brake fluid and noticed that the diaphragm on top in the master cylinder has a small tear. How serious is that?

McCabe-Thiele (Ted):
Not sure what the diaphragm is for -  to minimise moisture ingress or keep the fluid in place when cornering.

K2-K6:
It's for expansion space in hydraulic system when used/heated in operation,  if not there the pressure build would just squeeze the piston and apply the brake until it cools (cars vent to air as they don't ordinarily get too out of level like a bike cylinder) with the diaphragm being part of design concept to work effectively.

Fluid below is seperate from air above to prevent risk of air being near the fluid entry port to operating piston and bleeding air into line supply.

Needed to function correctly on any brake that's tipped up during normal use. Think about parking on side stand with  bars turned to the left.

Bryanj:
Depends where the tear is but they should be easily found as same one fits a lot of Hondas

robvangulik:
Just replace it, according to cmsnl this part fits 4875 models, an original Honda one costs a staggering €6,00, and the non OEM alternative is €2,50.
Gives you the opportunity to refresh your brake fluid at the same time, as you stated recently that's over 10 years old.
45520.300.000
At Roukama €7.00

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