K100RS Soft Front Brake

[bprigge]

Hello Everybody,

This weekend I changed fork seals in my 91 K100rs and ended up with a spongy feeling front brake lever and reduced front brake power, like I have air in the lines or calipers. I took the calipers apart to get them off of the disks during dissasembly. This may or may not have been necessary but I couldn't see any other way to get them off.

I bled the brakes as if they were ordinary car brakes but my procedure does not appear to have done the job as the lever is still spongy. Is there something I am missing here? This is my first exposure to Brembo calipers. Any advice appreciated, thanks, Bruce

Also my first exposure to ABS!

[bprigge]

Here is an interesting addition.

I just road the bike to work, about 50 miles on the interstate. The last mile or so is stop and go city driving. When I pulled into the parking lot my right rotor was cool to the touch, the left rotor was pretty warm as if the left rotor is doing all the braking work. Hmmm....? Is it possible to put a caliper together wrong? The front brakes still lack the power they had before I changed the fork seals and in spite of another bleeding drill. The second time I bled the front ABS too. Bruce

[mieczkow]

I did a complete rebuild on my K75S with Brembo stock brakes this year rebuilt the front and rear calipers, put on new rotors all around, put in stainless steel lines, total flush and clean of the system. Everything came out OK. Let me make a few suggestions about what might be going on and what might be worth checking out to try and solve the problem.

1. It is not necessary to disassemble the calipers as far as I can imagine to get them off the rotors. The calipers are bolted to the fork leg with two bolts. Remove these bolts and the calipers lift off intact, attached to the bike only by the hydraulic lines coming from the master cylinder. When you say disassemble the calipers, I assume you are talking about removing the two bolts which hold the caliper halves together. If you have taken these apart, then you need to check some other things.

2. If you popped out the brake pistons to change the rubber rings, it is possible that when you reinstalled the pistons one was not inserted properly and is now seized in the caliper. With one piston moving and the other stationary, you will get the "one side warm, one side cold" effect. If this has happened you will need to disassemble again, check to make sure the piston or caliper wall is not damaged, and reinsert the piston correctly. Also, since brake fluid is transferred across the caliper halves, there is a port with an o-ring which needs to be in place when the two halves are mated back up. It is possible that this is blocked and a block in this will also cause failure of the piston to operate on one side of the caliper.

3. Do not continue to drive the bike with the brakes working assymetrically. It is dangerous (because of reduced and improper braking force) and it will eventually have the potential of screwing up the rotor. Considering that front brakes provide the heavy lifting in stopping your motorcycle, you do not want to operate a motorcycle with problematic front brakes.

4. Bleeding the brakes should be pretty straightforward and very much like bleeding car brakes (I am assuming no ABS). I generally recommend speed bleeders because this makes the job very easy. When you are doing the bleeding make sure you do not "over compress" the brake lever and force the master cylinder piston beyond its normal travel range. Some folks recommend taping a little wooden block or something on the brake lever to stop the lever before you squeeze it to vigorously. You can also use something like a mightyvac device to suck fluid from the caliper end of the system - this works really well but you have to get a hold of or buy a vacuum pump. Also, be patient with the bleeding process - if you are pumping it through by using the "squeeze the lever" technique - it takes some time to get all the air out of the line.

5. It is always possible that some other aspect of the mater cylinder/piston/lines has been compromised - the master cylinder may have suffered some damage, etc. during the process that is responsible for the sponginess and the inability to get the brakes firmed up and normal. If you can't get it right, you'll have to bite the bullet and get it to a dealer or a reputable motorcycle mechanic - don't risk your safety by driving on shakey brakes!

6. I hope this helps some. Good luck!

[bprigge]

Tom,

Thanks for the reply!!

The calipers where 'split' by removing the 4 allen bolts that hold the two halves together. This bike has ABS and Brembo 4 piston calipers. (two on each side of the caliper) I did not remove the pistons from the calipers.

The second time trough I bled the ABS first and then the front wheel cylinders using the brake lever to pump air out.

On every bike that i've worked on there was enough room between the outer edge of the disk and the inside of the rim to just slip the calipers right off. Not so with this 91 K100RS. The bike has a 17 inch front rim, that may have something to do with the lack of caliper removal clearance. On this bike when you unbolt the two mounting bolts that hold the calipers to the fork leg and slide the caliper towards the rim you run into the rim before you clear the disk. I really didn't want to split the calipers apart but the only alternative I could see was to remove the disks from the rim and take off the calipers and disks off together.

I DID squeeze the brake lever all the way down to the throttle grip the first time I bled the brakes, possibly causing some seal damage according to a couple articles I found on the internet, and you also. That wouldn't account for 1 side working and the other side not working though. I guess i'll take the right side caliper apart again when I get home tonight and see if there is any visable problem.

The bike has about as much front brake now as my R75/6 had. If I jumped off the /6 and right onto the K100RS I probably would not have recognized the front brakes were weak on the K bike. I've put 2000 miles an the K bike in the past three weeks though and every time i've ridden it i've marveled at how smooth and powerful the front brakes were...until I putzed with it. Thanks again for the response, folks don't seem to like to talk brake talk here. Bruce

[bprigge]

After bleeding the front brakes 3 times with the old fashioned 'pump the brake lever' technique AND 600 miles of riding the brakes are now back to what they were before I took them apart. I think the 600 miles of riding did more for it then the extra two brake bleedings. Bruce