Technical Q and A for classic motorcycle maintenance and repair.
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Q: I am stumped with a front brake issue on my 1975 Norton Commando MkIII. I have always had a slight pulsing from the front brake near the end of my stops. It has the Norton/Lockheed master cylinder and caliper. The master is sleeved, and I replaced the old rubber lines with braided stainless lines. I had an issue with the piston in the master cylinder sticking when it got hot, like from sitting in the hot sun. The master got sent back to the shop that did the sleeve. They said they got a delivery of pistons that were not made to the correct tolerances. I think, but am not sure, they replaced the piston. The owner said they bench test every master cylinder before they ship them. I am confident of the shop and what they told me.
When I put it back on the bike, I had a terrible time getting any lever pressure, but eventually prevailed — almost. I noticed I have too much travel in the lever and it feels spongy. Since starting this repair, I rebuilt the caliper, sent the master back to have it checked again (they said it checked fine) and rebuilt the caliper a second time with new stainless pistons netting the same result. The bore on the caliper appears smooth.
I have tried every trick I know, and some given to me by an excellent mechanic, but cannot get a solid lever. I have bled almost a quart of fluid through the system and am confident there is no air hiding. I have put a tie wrap on the lever holding it about halfway in and left it overnight. The next morning, the feel of the brake is perfect. If I roll the bike out of the garage to test ride it, the lever travel dramatically increases and I can squeeze the lever to the grip. My last check was the disc itself. It shows a pretty consistent thickness all the way around. I checked for run out with a dial indicator. I found it to be as much as 0.028mm at one point.
So here is my real question: Is 0.028mm variance in the run out enough to push the caliper pistons back into the caliper since it is solid mounted? My theory is the piston may get pushed back into the caliper allowing for too much travel to give proper braking action. — Craig Bryant/Anaheim, Calif.
A: You’ve been very methodical in your testing and eliminated most of the easy answers I could have provided. I think you can probably do one more test and see for sure if the run out you find in the disc is the cause of your excessive lever travel. Find a way to elevate the front wheel while the bike is on the centerstand, enough so that you can spin the wheel freely. Pump the brake handle until you have the lever feel you consider normal. This next step may be easier with a helper: Remove the cap from the master cylinder and closely observe the fluid level while your helper spins the wheel. If you can see any significant increase in the level in the reservoir you can probably conclude that the disc run out is pushing the pistons back into the caliper.
Note: We received an email from Craig saying he found the problem and solution. It seems the previous owner had put a pre-MkIII brake lever on the bike. Once the proper lever was fitted, the brake lever worked as it should. MC