Pulsating Brake Pedal

[Laughing_Jakal]

While riding my Harley in Florida one day, I got the proverbial opposing traffic left turn in front of me. I got on both brakes hard....standing on the rear pedal and managed to stay upright. I avoided disaster, but pulled over and sat on the curb to settle down. When I rode the bike home I noticed that the rear brake pedal "had nothing". I pulled over and pumped that brake pedal and it got firm...thinking I had ingested air, I went home and was determined to bleed brakes.

When home, I pumped the brake pedal a few times and the pedal was as firm as it ever was. I then went to ride, and after I rode a few feet, the pedal was slack again. So I checked the reservoir (Full), then bleed the brakes...no air. I wasn't very mechanically inclined then and had no clue. I took it to the local Harley dealer and they said there was nothing wrong with the brake system....said that it was probably master cylinder had air in it. After paying their diagnostic fee, I found out that it was cheaper to buy the part than pay another service fee..

I figured I would change the master cylinder myself and got the same results with the new master cylinder.

I had the rear wheel on and off a few times, checking the caliper, (after re-aligning the rear wheel with the front using an 8' square tube aluminum extrusion per the Harley manual) I was sitting with my FIL ...an old motorcycle guy explaining the problem to him. He was also flummoxed as we had completely replaced all the fluid at this point and there were no leaks.!

While we were talking and drinking a beer, I spun the rear wheel out of frustration and I caught motion.....The rear rotor was so warped I could see the warp (Run out)....the runout was so severe on that skinny rotor that when the rotor spun with no pressure on the caliper piston, it would push the piston more than the distance it travelled when pressing the brake pedal one "throw"....when I pumped up the brake with multiple pumps, the piston would eventually cause the pads to make contact and it would temporarily unwarp the rotor.

When the brake was released and the rotor turned it would then push the pads far apart again. Which is why at a standstill I could pump up the brake and get it firm and it would then apply normally after pressure was built, but then after rolling a few feet, the pedal would go slack again.

Wheel off one more time, replaced the rotor (20 years ago it was $100).....haven't had a problem since. That rotor absorbed so much energy it warped.....it didn't owe me anything...lit saved my life and I learned a heck of a lot about motorcycle brakes.