Master Cylinder Priming Process

From: "Brian Mehosky
November 2001

This is based on only one (well, possibly two) data points, but it appears that priming and bleeding front brake master cylinders on ABS equipped K-bikes is a total PITA. In the hope that I may save some fellow rider/wrench similar problems, I thought that I would share my modest amount of experience.

I have a 1988 K100RS ABS Special. I noticed a smear of brake fluid on the bottom of the right grip assembly, and a month later the front brakes started to fade. I figured that it was time for a master cylinder rebuild.

That is not as simple as it sounds. There are two master cylinder assemblies available for 88 K-bikes; one for ABS and one for the non-ABS machines. Replacement pistons for the non-ABS units are available for about 22 bucks, but no similar assembly is available for the ABS units. But both are 13 mm pistons, so I gave it a try.

Be aware that 13 mm cylinders are very small. My honing rig would not fit inside the bore. Since I had it apart already, and the worst I could do was trash the cylinder, and the bore was in pretty good shape, I elected to use a brass brush (like that used for cleaning gun barrels) in a cordless drill to dress the bore ID. (I must tell you, that such practice cannot be endorsed or recommended by anyone, since these are your brakes. I'm just recounting what I did.)  I also used a wire wheel in a Dremel to clean out the snap ring groove in the cylinder. Then I cleaned things up and installed the replacement piston.

I did not prime the master cylinder on the bench, since I was concerned about getting brake fluid all over the bike. So I mounted the cylinder on the handlebar and connected the brake line banjo fitting loosely. Then I filled the reservoir, and pumped up the system and caught all the brake fluid leaking out of the fitting in a wad of paper towels. Things looked like they were working properly, so I snugged up the banjo fitting while pumping more fluid out of the system, in the hopes of minimizing the amount of air left in the system. Then I bled the system, at the caliper and at the servo.

No matter what I did, I could never get the system to pump up properly. I went through about a quart of brake fluid before I surrendered, and figured that I had used the wrong piston in the rebuild, or botched the rebuild in some other way. So I dropped the money for a new master cylinder assembly.

When the part came in, I repeated the mount/prime/bleed routine, and again I got no "pedal". I used another quart of brake fluid (by this time, I was grateful that I had purchased the gallon bottle), and even took the machine for a short spin, in the hopes of dislodging any air bubble. No success.

A plea to fellow wrenches on the internet got a reply asking if I understood how to prime a pump. That got me thinking. The system was behaving as if the master cylinder just was not pumping.

I purchased a rubber stopper, and dug a hypodermic needle and syringe out of my tool kit. (I have a selection of syringes I bought from a veterinarian supply house, which are great for measuring fork and shaft oils. The needles are a bit tougher to get, but make the best splinter removing tools around.) I poked the needle through the stopper, and filled the syringe with brake fluid. Then I stuffed the stopper into the bottom of the reservoir, opened my SpeedBleeder, and forced brake fluid into the system. I also pumped the brake lever while doing this, all in hopes of filling up the master cylinder. Then I bled the system again.

That got me about seventy percent of my front brakes back. A short ride, followed by another bleeding, and the brakes were back where they were supposed to be. Surprisingly, I never saw any bubbles when bleeding the system. I suspect that the air migrated back to the reservoir.

I haven't gone back to see if the rebuilt cylinder would work, if properly primed. It may have been fine, all along.