Front brake master cylinder question

Steny365 · May 19, 2019

Hi folks looking for advice , have a 08 GS ,when I pull my front brake lever I can hear a hissing sound of air wondering if this is normal ,I don't have servo brakes , I'm thinking maybe my master cylinder is on its way out , cause sometime not all the brake lever would go in a far bit but Harding up when I pump , brakes are bleed but as I have said I'm hearing a hiss/air noise when I pump the lever and seems to come from cylinder,no leaks anywhere to be found ,

Anyone have this issue before that lead to it been the master cylinder or I should noise normal and maybe I still have air in system,

NicD · May 19, 2019

Are there any symptoms such as soft lever, lever back to bar, fluid leak under m/c or anywhere else, no fluid in reservoir etc? Have you just replaced fluid or bled brakes, changed pads etc?

Steny365 · May 19, 2019

Hi NicD

I haven't changed anything I bought it a few months back and always made that noise , but one time while In shed sitting on bike I pressed the lever and it went in nearly all the way but I pumped it and it went hard again ,it hasn't happened since , I checked everywhere for leaks no sign and plenty of fluid in bottle , just getting paranoid now if you get me as wouldn't like that to happen while driving , so I was thinking if that hissing noise isn't normal I'm wondering if the MC is starting to go like one of the seals maybe

Are the brakes linked on these the front and rear

NicD · May 19, 2019

Hard to describe noises accurately, but some low level 'pssss' is normal when you pump the lever.

Yes, front applies small amount of rear on my abs bike. don't know for non-abs, and not sure which you have.

If m/c leaks you would expect fluid seepage or air getting into the system, in which case lever will go soft as air compresses whereas brake fluid doesn't.

If you test the brakes and the bike stops normally, there are no symptoms at lever, and no fluid leaks, I would assume the noise is the same noise I can hear on mine (when eingine off and no helmet on).

But like i said, hard to know from description and it's a judgement you need to make for yourself

botus · May 19, 2019

I would say its normal, mine did it for many years, later bikes with ABSII , and yes they are linked

going back to the bars is interesting.... if you had pushed the front pads back into the caliper a bit and or someone had pushed against the rear sliding caliper (…. pressure applied from right to left as in leaning against the rear caliper can do it...) in fact on all three calipers regularly moving the pistons a significant amount and pumping them back is a good idea to help keep thing moving

Steny365 · May 19, 2019

Hi botus

Thanks also for the reply , yeah maybe your are right noise is Probaly normal , the lever pull is grand but once or twice it did go soft that impression you get as if you did move back the pads ,then it would go hard ,hasn't done it since ,reason I asked if rear is linked ,a few weeks back I noticed some wetness around the rubber cover on the rear master cylinder near the adjust nut ,I wiped clean and have pumped rear lots of time and no more leak , so I'm thinking of there linked and there is a small leak in rear cyclinder could it give a soft feel in the front lever ,

Also does the link only work with bike running cause with brake applied when not started the rear wheel moves freely without any resistance,

Anyway I have a BMW shop looking into it Tuesday for me to also fully bleed the system and flush

botus · May 20, 2019

Steny365 said:
Hi botus

Thanks also for the reply ,a few weeks back I noticed some wetness around the rubber cover on the rear master cylinder near the adjust nut

what bike? 2007 one's had a recall for fracturing brake pipes, but your point could be collection of rain, oil lubricating the pivots / linkage or indeed a weep from the master cyl

uncle buck · May 21, 2019

Mines a 09 and have the "hiss" so to speak. Never had a problem and no leaks. Doing fluid changes this week, so will check it out on mine.

Front brake master cylinder question

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