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1991 K100RS - Brake Problems, Help!

C

charlesdonahue

Guest
I recently purchased a 1991 K100RS with ABS from a private owner.

While riding it home after purchase, I noticed that the front brake lever would feel really stiff when I pressed it. Furthermore, it feels like the front caliper would bite the rotor when the brake is squeezed, but is very slow to release the rotor. In some instances, if I use the front brake, then pull in the clutch, the front brake seemed to continue braking, even though I was not pressing the front brake.

Thought some clean brake fluid to the front end would fix it. Bled the front brakes with a pressure bleeder, added fresh fluid… but still having same problem. Friends and family are suggesting it's either ABS problem or calipers need to be rebuilt.

Well, today I was riding the bike and another fellow biker flagged me down. Told me that my rear brake light was always on. Bad news. So now it looks like there could be 2-3 different things going on causing this front brake problem. Before I go and take it to the BMW dealer and pay them too much money… anyone have any ideas what could be causing this, or what needs to be fixed?

Any insight here would be GREATLY appreciated! Thanks so much all!
 
It sounds like either the front brake lever is sticking or the piston in the master cylinder is sticking.

There is a switch at the front lever that actuated the light whenever the lever is not fully returned. There is a spring on the piston in the master that should return the piston. The piston should push the lever back to its returned position.

Try turning on the key and the kill switch, hit and release the rear brake, then hit and release the front brake. See if the brake light is still on. If it is, lift up on the rear brake pedal. Does the light go out? No? Now push the front brake lever forward. Was there ANY resistance? Did the brake light go out? Is there ANY free play between the returned position and where it contacts the master cylinder piston?

Get back with your results.


:dance:dance:dance
 
It sounds like either the front brake lever is sticking or the piston in the master cylinder is sticking.

There is a switch at the front lever that actuated the light whenever the lever is not fully returned. There is a spring on the piston in the master that should return the piston. The piston should push the lever back to its returned position.

Try turning on the key and the kill switch, hit and release the rear brake, then hit and release the front brake. See if the brake light is still on. If it is, lift up on the rear brake pedal. Does the light go out? No? Now push the front brake lever forward. Was there ANY resistance? Did the brake light go out? Is there ANY free play between the returned position and where it contacts the master cylinder piston?

Get back with your results.


:dance:dance:dance


As soon as I turn the key and flip the kill switch, the brake light is on as if braking. Pressing front and rear brake don't turn it off. While pressing on the front brake it is very stiff. Very small amount of game as your pull before it gets stiff.
 
What happens when you push the lever FORWARD to the stop? Does the light go out?

Does the light go out if you lift up on the rear brake pedal?

One or the other SHOULD make the light go out.



:dance:dance:dance
 
The reason for it being very stiff is your front master cylinder piston isn't returning all the way out. It has to do that to open up the pressure relief hole in the master cylinder that depressurizes the brake circuit. Same reason the brake light is staying on.

WAG - you're looking at a new master cylinder, unless the lever is binding for some reason (which should be pretty obvious.)

With a 20 year old bike - this sort of failure isn't a surprise, especially if the brake fluid wasn't flushed on the called for 2 year schedule.
 
What happens when you push the lever FORWARD to the stop? Does the light go out?

Does the light go out if you lift up on the rear brake pedal?

One or the other SHOULD make the light go out.



:dance:dance:dance


Pushing front brake lever forward, brake light is still on.

Pulling the rear brake up and brake light still on.

I think I'm just going to take it to my local dealer and have them take a look at it.
If it's a master cylinder problem, I don't know much about how to go about fixing it :cry
 
Pushing front brake lever forward, brake light is still on.

Pulling the rear brake up and brake light still on.

I think I'm just going to take it to my local dealer and have them take a look at it.
If it's a master cylinder problem, I don't know much about how to go about fixing it :cry

it's a replacement, not a "fix". should really not be that difficult to do yourself. guess it depends upon where you like to throw your $.
 
Well… I would prefer to do the work myself. I just don't have as much experience as I would like as far as working with the brake system. Don't want to do more damage than good. I can't imagine it's that difficult to replace the front master cylinder though....

I priced a few front master cylinders on ebay. Thinking I'm going to do a bit of reading in my Haynes service manual, and see if it's something I could do in my own garage.

Any suggestions on how I can confirm that the problem is being caused by faulty front master cylinder? I would really hate to drop $100 on one and find out that I don't need it :/
 
Before you go replacing the master cylinder, remove the front brake light switch. It is possible that the plunger on the switch is stuck out (in the on position) and that is keeping the lever from fully returning.

Yes, it's possible like you suggested earlier, that you have more than one problem, but start with the easy and cheap one first: the brake light is on.

If the switch plunger is stuck, try to clean it to unstick it or just unplug it under the gas tank and see if the light goes out.

61 31 1 459 569 SWITCH $34.42

If the switch is not stuck and it appears to actuate properly, try unplugging the rear brake light switch. Plug is on right frame tube under the battery cover just in front of the coolant bottle and just below the gas tank (two conductor). See if the light goes out.

Once you figure out why the light is on (and it has to be one of the two switches is either stuck or the lever or pedal is not fully retracted) then it will be easier (and cheaper) to diagnose the brake problem.



:dance:dance:dance
 
Lee,

He said "In some instances, if I use the front brake, then pull in the clutch, the front brake seemed to continue braking, even though I was not pressing the front brake."

To me - that yells front master cylinder. Can't see a coincidence where the rear switch failed and the front master cylinder failed at the same time.

Charles,

As costly as it might be - I'd avoid the ebay used master cylinders. Chances are they are almost as worn out as yours - and you have no idea until you install one if it even works correctly. To me brakes are important.. It may be possible to find a rebuild kit for your master cylinder - but it's also quite possible that the bore of the cylinder is pitted, in which case, the rebuild isn't going to last.

It's a case of sucking it up and buying a new one.. at least for me..

To see if the master cylinder isn't returning all the way is pretty simple. Unscrew the top cover of the reservoir (after covering anything under it that fluid could drip on) - and pull the lever and release it. If the cylinder is working correctly - there should be a little spurt of fluid appear about where the working end of the cylinder is under the reservoir. That's the relief hole, and the spurt is caused by pressurized fluid flowing back into the reservoir (it gets pressurized when you use the brake by the hoses slightly expanding).

No spurt - chances are good the piston isn't returning all the way. You might try removing the lever and looking at the end of the cylinder. Often there is munge building up around the open end of the cylinder due to slight leakage through the seals and crap being absorbed by the dampness.

There is a good reason BMW spec's changing the fluid (flush) every 2 years. The fluid absorbs moisture from the air and the moisture can cause corrosion of the inner working bits of the brake system. It also can cause a loss of brakes if the fluid gets hot enough to boil the moisture (fluid isn't terribly compressible, steam IS..) If the system has been neglected, it's really time to go through it and make it safe again. To me - that would be a new master cylinder and a complete flush of the system right through the ABS and down to the calipers.

Good luck!
 
He said "In some instances, if I use the front brake, then pull in the clutch, the front brake seemed to continue braking, even though I was not pressing the front brake."

To me - that yells front master cylinder. Can't see a coincidence where the rear switch failed and the front master cylinder failed at the same time.

When I fully squeeze front brake lever, the front brake closes onto the rotor. While parking for instance, as soon as I press that front brake lever it feels like the calipers are biting onto the rotor, and don't want to let go. I have to rock the bike back and forth in order to get the front calipers to loosen up on the rotor.

To see if the master cylinder isn't returning all the way is pretty simple. Unscrew the top cover of the reservoir (after covering anything under it that fluid could drip on) - and pull the lever and release it. If the cylinder is working correctly - there should be a little spurt of fluid appear about where the working end of the cylinder is under the reservoir. That's the relief hole, and the spurt is caused by pressurized fluid flowing back into the reservoir (it gets pressurized when you use the brake by the hoses slightly expanding).

When I bled the front brakes and added new brake fluid (which looked pretty clean while changing), with top cover of reservoir off and pulling lever there was a little (and sometimes large) spurt of fluid squirting out of othe reservoir. It sounds like your telling me that would suggest there front master cylinder is working properly.

Before you go replacing the master cylinder, remove the front brake light switch. It is possible that the plunger on the switch is stuck out (in the on position) and that is keeping the lever from fully returning.

If the switch is not stuck and it appears to actuate properly, try unplugging the rear brake light switch. Plug is on right frame tube under the battery cover just in front of the coolant bottle and just below the gas tank (two conductor). See if the light goes out.

When I get home from work, this is my first plan of action. I am going to try unplugging each of the brike line switches, see if I can find what is telling the brake light to be on... then go from there
 
Lee,

He said "In some instances, if I use the front brake, then pull in the clutch, the front brake seemed to continue braking, even though I was not pressing the front brake."

To me - that yells front master cylinder. Can't see a coincidence where the rear switch failed and the front master cylinder failed at the same time.

Don:

Like I said, he might have multiple problems, but one problem for sure, is something is actuating the brake light (even when he pushes forward on the lever). What ever is holding the lever from returning far enough to turn off the brake light COULD also be keeping the lever in the applied position relative to the master. That COULD be easier AND CHEAPER to find. Then if he STILL has brake issues, he's only chasing one issue.

The switches are normally closed switches. No pressure on the switch=brake light on. Push in the plunger on the front switch=brake light off. The rear (after 9/86) is a microswitch with activating lever arm. No pressure on arm=brake light on. Pressure on arm=brake light off.

If he could get the brake light to go out by gently pushing forward on the lever against the switch, then it would no doubt be the master, but the fact that the light does not go out suggests that there is (also) a problem external to the master (sticky lever, stuck switch plunger [I've had one on the rear of my bike. Prior to 9/86, they used the same switch front and rear. They probably changed the rear design because water and grit would cause them to bind].

Charles:

Also pull the pivot pin for the lever and remove the lever. See if anything is causing it to bind. Should be floppy loose on the pin and inside the perch. If not, clean it up.

Fix one problem at a time. Easiest, most obvious first (the light). Then move to the next problem.



:dance:dance:dance
 
So I took the cover off the underside of the front brake lever. After taking this cover off, I see the thin piece of metal that clicks on the little white button that tells the brake light to go on and off.

Turns out, that little piece of metal wasn't making correct contact onto that little white botton/switch. Bent it back into shape, put the cover back on, and brake LIGHT problem fixed.

So... I thought, maybe if I was lucky, that would fix the brake problem all together (feeling like caliper sticks on rotor). Well, sure as could be... rolled the bike off the centerstand, started the bike, and rolled around a bit in my garage. Front brake seems to release off the caliper as soon as I release the lever!!! :dance:dance:dance:
 
Oops! I was thinking about the 2v K100 front brake lever and brake switch.

I wasn't aware that they had changed the 4v front brake switch to a similar type as the post '86 rear switch.

The bent up piece of spring steel switch activator was not allowing the lever to fully return.

Like I said , check the simple/cheap things first.

Now , go riding!!!



:dance:dance:dance
 
You should also check the condition of the rubber brake lines. They can delaminate internally and actually act like a residual pressure valve. This would cause your brakes to drag.
Jeff
 
You should also check the condition of the rubber brake lines. They can delaminate internally and actually act like a residual pressure valve. This would cause your brakes to drag.
Jeff

Uh, I think he found the problem already.


:dance:dance:dance
 
Weather in Illinois is GORGEOUS today so I plan to take a ride after work to confirm that it's fixed. Sure felt like it just rolling around in my garage....

fingers crossed :thumb
 
Took the bike out for a ride yesterday and front brake is still locking :doh

I think I'm just going to take it to my local dealer this weekend and have them take a look at it. Make sure it's right and tight before I put all those miles on it this riding season.
 
I would still take a good look at those brake lines unless you know the history on them. A bike of that age should have the lines replaced by now but maybe they are still the originals.
 
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