Master cylinder failure

paddy o'busa

Hey guy's ,this ever happened to your bike?..... Went for a long road trip - three day's in i noticed my brake lever was getting a lot more travel than it used to , but wasn't sure a this ponit if i loosing braking power. The other guy's in the group told me that my brake light was staying on. Later that day i was definately getting less braking power and my lever was going all the way to the handle bar.
I'm fairly confident my master cylinder is failing. My brakes are'nt grabbing, and there's new fluid in there ( dot 4 ) which i know is fine. I can duplicate the problem standing beside the bike and pumping the lever ( more-so just after a ride ).

Have any of you guy's replaced a master?
How about a rebuild? The piston is probably alluminum or plastic and the seal is a synthetic rubber. Can't be that expensive? Just replace the whole piston assembly. Or if you have the $, try a new Brembo or Nissin radial master with remote reserviour. Very cool!
Sounds good Sierra Nevada, Do you happen to know of anyone who's tried replacing the master and was happy with the results? :super:
Not directly, but I was seriously considering changing my master for a better looking race type master. The key is to not change the brake geometry too much or you might get some new "personality" to the bike under heavy braking. What I mean is that some master cylinders have a higher ratio geomtery lever than stock. This means that the same pull on the brake lever will result in much higher squeeze on the brake pads and may end up too aggressive for street riding. During normal braking you may accidentally pull to much front lever and lock it up causing a low side fall. My bike could use a little more than it has now. I would have to grab a complete handfull of stock lever to get the front out of shape. But I like to use two fingers normally and can't get that much pull. The ratio is determined by the distance from the pivot to the the brake piston compared to the distance from the pivot to the middle of the lever where your fingers rest. The higher that ratio is - the more leverage your grip has to pressurize the brake fluid and the less force needed to squeeze the pads. Too little and brakes are ineffective. Too much and you will loop it forward when barely grabbing the front lever or lock it up and slide out in a turn.

Nissin and Brembo offer a 17mm and 19mm diameter piston. The larger one pushes more fluid per mm of travel. It takes less lever movement to activate your brakes given the same size caliper poistons. The The size of the caliper pistons really determines how much diameter you need on the master piston. Larger calipers need more fluid. Smaller master pistons will have to travel farther to pump that much fluid or you can increase the diameter and decrease the travel.

Of the two, The leverage ratio is most critical. A small change in this pivot placement can result in a big difference in brake feel. So if you choose to go this route, take care to pick a master with similar geometry to stock with a small increase (if any) to that ratio. The diameter is not so critical. larger diameter will mean less lever movement. It will feel stiffer but not give any more braking power.
Something else you might check for while your at it is hose bulging. Worn lines can stretch under pressure, might be time for some new ones, or steel braided lines for some good stopping power.