I recently needed to replace the front brake pads on my bobber. While doing so I figured it was a good time to go over the rear brake as it had been a little spongy and the pedal was bottoming out at the end of travel. I found the master cylinder leaking so decided to rebuild it.. It was leaking mostly from the outer edge of the disk that covers the master cylinder components so I figured to make a mount for some rubber boots that I'd bought some time ago.. Here are some pics of the boot and the master cylinder.








The end cap for the parts had a o ring on the inside but none on the outside.. I was thinking about using X type O rings but ended up with a simple round O ring. I decide to machine and O ring groove in the outside to help keep crap out of the master cylinder.. I was thinking that this was the cause for the leak but later figured out I had other issues.









Cont...

 

I started rebuilding the master cylinder by running a hone through the cylinder and found 2 severe divots in the cylinder that were from the master cylinder piston cocking on the bore so the bore was pretty much shot. I looked for some other masters but most custom ones were pricey and I'd have to make mods for them to work. I started to think about sleeving but the sleeve would have to be about 0.025 thick for the end cap to work. I got to thinking. The original cylinder was 3/4" and required a bit of pressure to get the rear brake working. Maybe I could find a rebuild kit with a smaller diameter piston and decrease the pedal pressure required on the rear brake.. I found a 5/8 rebuild kit on fleebay that look to have the same internals as the MC I have but 5/8 piston, seals and spring..

I found a precision bronze bearing with OD +0.002 over 3/4 inch and the ID set to 5/8 that was long enough from McMaster. The original cylinder after hone was 0.751 so I had 0.001 interference fit..

Here is a pic of the 2 setups.




One cool thing about the 5/8 kit was that the right bellows seal for the reservoir..

Heated the master cylinder and froze the bearing sleeve , then pressed the sleeve into the master..

 

I then drilled the small holes through the sleeve that need to go from the reservoir too the cylinder..





I assembled the master cylinder and bled the brakes. Test ride showed that the rear brake worked a bunch better..





I'll see if it leaks.. Hopefully it won't.. While doing this rebuild I think I figured out what caused the wear. The rod going into the master cylinder needs to compress the return spring a little. The preload keeps the brake pedal from rattling and that rattle caused the rod to make the piston rattle in the bore wearing the bore.

 

good job