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Rear Brake - Hard/Spoft Pedal Mon Sep 16, 2019 11:32 am
cycleman
Silver member
I recently bought a 1986 K100RT and have been slowly going through some of the things that go bad with a 33 yr old bike that has sat for most of its life.
With the rear brakes I had replaced the rear master cylinder and rubber brake line. The old master had pitting. After numerous attempts to get a hard pedal, the symptoms where that I could bleed the brakes get a hard pedal and then slowly rotate the rear wheel, apply the brake periodically, then all at once the rear pedal would go down about twice as far. For the life of me I couldn't figure out what the cause was as I'd replaced everything in the rear brake system including rebuilding the rear caliper. I kept thinking that I had air somewhere in the system.
To make a long story short, I pulled out my dial gauge and set it up on the rear disk. When I slowly turned the rear wheel and watched the dial gauge, I could see that it moved about 40 thous. I could match up the 40 thous with the soft pedal. I checked on the stock run out, which is 8 thous. So I found the problem the rear disk has a spot that is out of true and outside the tolerances. Rear wheel is fine and within specs.
The soft pedal was a result of the brake pads having to travel further at one point on the disk, which required more fluid to fill the cavity behind the pistons and the pedal had to be pushed further in its stroke to accomplish that. When you drove it that way you always had a soft pedal with no rear brakes, but at a stop you could pump the rear pedal and get brakes, that stayed hard until you moved the bike and then the brakes would go away.
Posted on the oft chance that somebody is having or had similar issues. New disk & pads on order. Too bad as the old disk had minimal wear as the bike has low kms.
With the rear brakes I had replaced the rear master cylinder and rubber brake line. The old master had pitting. After numerous attempts to get a hard pedal, the symptoms where that I could bleed the brakes get a hard pedal and then slowly rotate the rear wheel, apply the brake periodically, then all at once the rear pedal would go down about twice as far. For the life of me I couldn't figure out what the cause was as I'd replaced everything in the rear brake system including rebuilding the rear caliper. I kept thinking that I had air somewhere in the system.
To make a long story short, I pulled out my dial gauge and set it up on the rear disk. When I slowly turned the rear wheel and watched the dial gauge, I could see that it moved about 40 thous. I could match up the 40 thous with the soft pedal. I checked on the stock run out, which is 8 thous. So I found the problem the rear disk has a spot that is out of true and outside the tolerances. Rear wheel is fine and within specs.
The soft pedal was a result of the brake pads having to travel further at one point on the disk, which required more fluid to fill the cavity behind the pistons and the pedal had to be pushed further in its stroke to accomplish that. When you drove it that way you always had a soft pedal with no rear brakes, but at a stop you could pump the rear pedal and get brakes, that stayed hard until you moved the bike and then the brakes would go away.
Posted on the oft chance that somebody is having or had similar issues. New disk & pads on order. Too bad as the old disk had minimal wear as the bike has low kms.
