BMW K bikes (Bricks)

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1Back to top Go down   Rear Brake - Hard/Spoft Pedal Empty Rear Brake - Hard/Spoft Pedal Mon Sep 16, 2019 5:32 pm

cycleman

cycleman
Silver member
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.

    

2Back to top Go down   Rear Brake - Hard/Spoft Pedal Empty Re: Rear Brake - Hard/Spoft Pedal Mon Sep 16, 2019 5:54 pm

92KK 84WW Olaf

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Life time member
Life time member
When you pull off the disk check it.......a slight possibility it as not mounted fully flat and is not warped. I had a job to get my front discs to sit true on my RS, they were showing too much run out and eventually had to clean off the mounting points a bit to get them running true.


__________________________________________________
1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 42,640 miles.

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

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