85 K100RT Rear brakes

[roy]

On my bike the rear disc brake unit will not even attempt to lock up the rear wheel on dry pavement. Is this normal?

1) I have bled the brake and it made no difference. Brake pedal is not spongy but does not feel rock solid like other squid bikes I have owned.


2) Should I consider replacing the master cylinder or is their a rebuild kit available? I looked through the Tech section on K-bikes and could not find anything.

Thanks

Roy

Did I buy a money pit? or is this post buyers blues :confused:

[sgborgstrom]

Did I buy a money pit?

Define money pit....

What year is the bike? Could it be your ABS kicking in? What about condition of the brake pads and rotor. New pads take quite a while to "bed" to an old rotor especially if it's got much grooving. Does the rear brake work at all? Is there some slippery crap on the rotor from when the old owner was cleaning it up for sale?

The above is where I'd start before getting into new master cylinders and stuff.

Steve

[kbasa]

If I remember correctly, the caliper is lower than the master cylinder on that bike. That creates some interesting bleed problems.

Paging Don Eilenberger. Mr. Eilenberger to the white courtesy phone.

[2whelman]

I have the same bike and it should be able to lock up the rear wheel. My bikes master cylinder rotted out at about 10 years, and then again last year. I replaced entire master cylinder, flushed everything each time and have no problems. It is not as strong as a rear brake as other bikes I have had, but it should have enough power to lock it up, though it takes a good stab at the pedal to do it. My front master cylinder finally gave up this year too. Not bad for 20 years of use. Every time I think I am putting too much money in my 85k100rt I just look at the price of a new one. Still makes sense to me to keep the old girl on the road. Hope this helps.

[sgborgstrom]

My bad about the year of the bike, "title" of the thread doesn't show up in the reply section.... :hide

Steve

[deilenberger]

If I remember correctly, the caliper is lower than the master cylinder on that bike. That creates some interesting bleed problems.

Paging Don Eilenberger. Mr. Eilenberger to the white courtesy phone.

Ah yes! Hi Dave :clap Actually - the master cylinder is located on the bottom edge of the foot-peg plate below the axis for the swingarm, and the caliper is located on the top of the swingarm, so bleeding is pretty much normal.

Ok - to the question at hand - this is ABNORMAL. I could very easily lock up the rear wheel on my FYK ('85 K100RT), which is the reason I eventually converted it to ABS (you don't want to do this.. believe me on this..)

Something is wrong, so lets do some elementary troubleshooting:

1. Get something nice to sit on since you'll be staring at the rear drive for a bit and squatting just doesn't hack it. A stool about 10" high is good.

2. If you have luggage/bags - remove them.

3. Look at the rear brake rotor. Is there a big lip around the outside edge? If so - it is badly worn, but this still shouldn't cause this symptom.

4. See any oil on the rim or the rotor? If so - rear seal has probably gone bad and you now know why the brake isn't working too well.

5. When you use the brake - does it feel firm? If not - it needs bleeding, which isn't all that difficult (and the master cylinder is lower than the caliper normally, so this makes it easier.) See your Haynes/Clymer manual for details on bleeding it.

6. Have you removed the pads and looked at them? Perhaps considered replacing them? We don't know how old these pads are. Brake lining can apparently become less effective with age.

7. Have you flushed (which means bleeding without the system ever going "dry") the brake fluid? BMW recommends this be done every 2 years.

If it isn't one of the above - then it is something involving the hydraulics. If you aren't comfortable doing brake hydraulics - it might better be left to a shop (brakes are kinda important to me..) If it was me (and I'm not suggesting <b>you</b> do this) and I'd eliminated the obvious, I would look at:

1. Caliper - I'd be checking it for free piston movement. If they didn't move correctly, I would consider replacing it with a good used one.

2. Brake hoses - I would change this as a matter of preventitive maintenance. The hose is 20 years old.

3. Master cylinder - I'd be thinking about replacing it.

HTH, and with more details we might be able to provide more guidence.

Best,