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1980 R100 front brake free play

lmc3881

New member
I am completing a frame on restoration of a 1980 R100. I am having trouble with the front brake free play. The front wheel turn with just a little drag, as does my 1984 R100. The master cylinder cable has what I believe is the correct free play, yet I have almost no free play at the lever. The only way I can accomplish free play at the lever is to loosen the cable to the mc too much, as in the cylinder at the end of the cable isn't even touching the master cylinder lever that pushes the plunger. The mc seems to be working right as fluids exiting and returning are evident when I take off the cap and look into the reservoir. Anybody have any ideas?
 
Going back from memory, there is a special feeler guage from BMW to set the proper gap so I don't think it's meant to touch.

Hope this helps!
 
Ref., 1.20mm = 0.047"

BMW-feeler-tool.JPG


Being a /5 kinda guy I don't have a master cylinder. It's hard to tell from that old B&W manual photo; mechanically what's going on here, does the "fork" fit around the machined groove (holding it in place), and the "slack" is taken up/let out with the ferrule?

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My Haynes says:

"Slacken the cable adjuster locknet and insert the forked feeler gage into the groove in the master cylinder piston. [Note: the feeler gage is 1.2mm thick.] Adjustment is correct when the feeler gage is just freee to move against the master cylinder body."

So, if there's too much tension on the cable, it pushes the small cylindrical shaft which pushes the piston farther into the m/c body and the groove moves with it, eventually disappearing inside the m/c body. Too little tension, and the groove comes too far out. So, the free play/tension has to be just right so that the groove is near the m/c body but just near enough to fit the feeler gage into the groove.
 
I can see how that would be p-r-e-t-t-y hard to do without the special "feeler gauge".

Thx Kurt.
 
Thank you all for your replies. The problem, for some reason, is not with the piston position. Even if I slacken the cable so the barrel end is not touching the master, I still don't have enough free play. The only way I can accomplish free play is to loosen the cable so much that the piston can actually fall out. This leads me to believe the problem may be related to the calipers, but I can't identify what that might be.
 
The piston should not be able to fall out of the master cylinder unless the snap ring at the rear of the cylinder, as shown in the picture, is missing from yours. If the snap ring is there the piston will stop at the back of the cylinder and loosening the cable a bit more will only introduce a bit of free play in the system. If there's too much freeplay there is a danger that the cable will disconnect from the lever.
 
He might not be talking about the piston...probably the oblong shaft that is between the external lever and the internal piston. That's the gold cylinder object shown in Lew's picture.
 
Kurt is correct

Yes Kurt, you are correct. The piston is in place with the snap ring. If I back off the cable far enough to get free play then the piston plunger is in danger of falling out. Still perplexed, and missing some really nice California riding weather
 
The only way I can accomplish free play at the lever is to loosen the cable to the mc too much, as in the cylinder at the end of the cable isn't even touching the master cylinder lever that pushes the plunger.

Just throwing this out on the table...

A part number search for the rod ( 34 31 1 233 309 ) shows it's the same back to the /6 "to" 09/80, but it also shows up in '84 and '85 model years. So it's probably not the "wrong" part.

- Maybe you'se gots an aftermarket brake cable that's a tad too short?

- Short of that (ooops, sorry for the pun) is there any crud in the depression in of activator arm that might be taking up space (in effect "lengthening" the rod)?

- Is your brake lever solidly mounted, i.e. no slop (as a result of the bushing (#5)) being shot?

- The control perch ferrule is screwed all the way in?

- What's up with the "sleeve" (#20)? I don't know what it's supposed to do, but if it's sticking out too much it looks like the effect would be to "shorten" the cable

Logic check...

With the circlip in place the piston can't come out any further, correct?

You could tune the length of the actuator rod to shorten it a tad.. . .. . at your own risk of course. Personally, I'd want to know "why" the set-up isn't working correctly.

B0000667.png


Any crud built up inside the adjuster?

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In your first post, you made it seem like your calipers were adjusted correctly, that is, not dragging to any extent on the disk. Is that correct, are the calipers adjusted?

If so, and the problem is really not enough free play in the cable, Like yourself, I'm having a hard time imagining what is causing it.

The front brake isn't like the clutch, where you have two adjustments. The adapter sleeve on the brake hand grip doesn't have any threads, so no adjustment at the lever. The only threaded adjustment is on the master cylinder end of the cable.

I've never experienced the front brake cable adjustment needing to be "too precise". Since the adjuster is right there at the MC, you can eyeball the groove in the piston if you don't have the feeler gauge. Pretty close seems to be ok.

So, is it possible you have the wrong cable, like a short one for lower bars and you have high bars, so the lack of free play comes from the cable simply being too short?

Or, could you have the barrel inside the lever (or at the MC end) reversed? If the barrel was backwards, the cable end wouldn't slide into the barrel and you would loose 3-5mm of cable length.

Barron
 
The front brake isn't like the clutch, where you have two adjustments. The adapter sleeve on the brake hand grip doesn't have any threads, so no adjustment at the lever. The only threaded adjustment is on the master cylinder end of the cable.

I'm not speaking with any authority on this Barron, but the parts fische (MAX) shows an adjuster at the handlebar end and the master cylinder end. Granted, it seems redundant and kinda silly. I can't find a clear shot on the net. Maybe it is the MASTER CYLINDER adjuster (shown with the control perch (?)) #!7

Just looking through Hucky's collection of brake cables (there are a ton) between the R90 series and R100 they vary in length from 101cm to 112cm. It wouldn't be too much of leap to find out the OP's cable is incorrect. Typically, the amount of "free end" end between the ferrules is the same, regardless of overall length. :dunno
 
Thank you all for your replies. The problem, for some reason, is not with the piston position. Even if I slacken the cable so the barrel end is not touching the master, I still don't have enough free play. The only way I can accomplish free play is to loosen the cable so much that the piston can actually fall out. This leads me to believe the problem may be related to the calipers, but I can't identify what that might be.

Have the calipers ever been rebuilt? Are they Brembo calipers or ATE's? I had a similar issue with the ATE's on mine (single piston), but I had quite a bit of drag at the disks. Rebuilding the calipers with new rubber & pistons helped a lot. I think the Brembo calipers are two piston (each caliper), which would imply to me that they should have very little to no drag at the disk(s)/wheel. Maybe it's time to buy a couple of rebuild kits? Also, just to clarify, exactly how much free play do you have at the lever? Mine has about a centimeter...
 
Kurt, Lmo1131 and Barron

Thanks for sticking with this. To Kurt, the push rod is the same on both ends so no help there. My MC looks like one in Lmo1131 photo (without the rust). The only way for me to achieve free play is to slacken the cable so the cable end is not secure to the MC fork. (hopefully that makes sense) If I do so, then the brake lever is just free and doesn't spring back into the proper position against the lever housing. One thing I haven't considered, and am not sure why it would matter, is that I changed handle bars from stock. The stock bars are in a really silly position, somewhat like a beach cruising bicycle. The bars I put on are different, but not low bars. If anything that would make the cable too long. Is that possibly the problem? If so, how does one know what length it should be?
 
Jad01

I am not sure about the calipers, at the moment. I assume they are stock as the bike only has 24,000 miles. They are single caliper though. Maybe a rebuild is in order but it seems early to consider. I have 4 mm with the feral against the mc fork. I think the goal is 8-12.
 
Jad01

I thought I replied to the thread but it may be in space. I have a single puck caliper. I assume they are stock. The bike only has 24,000 miles so it hard to think a rebuild would be in order. I have 4 mm of free play with the feral against the mc fork. I think the target is 8-12.
 
I am not sure about the calipers, at the moment. I assume they are stock as the bike only has 24,000 miles. They are single caliper though. Maybe a rebuild is in order but it seems early to consider. I have 4 mm with the feral against the mc fork. I think the goal is 8-12.

My sense is that there are somewhat limited options for free play in this system (based on my experience with my undertank set-up). If you use the feeler gauge method that Lew references, the free play is pretty much constant, and would only change as the cable stretches over time (thus increasing play). My bike only has the adjuster at the master cylinder as Barron suggests- no adjustment at the handlebar lever.

It sounds like you have the ATE caliper (not the Brembo caliper). As Barron suggests, adjusting the caliper via the eccentric bolt is important to the performance of the brakes- if you have had the caliper out and did not properly adjust it when you reinstalled it, that could explain the lack of freeplay as the pads may no longer be parallel to the rotor. Otherwise, there is likely something at issue in the hydraulics. The ATE calipers are the most likely culprit in my experience (limited as it is). They are aluminum with a steel piston. The calipers are very prone to corrosion if the brake fluid is not changed frequently (annually). This is not a mileage issue, but rather one of time. As the internal surface of the piston and caliper bore corrode, and the rubber seal on the piston hardens, the piston does not retract as much in the bore when the lever is released and pressure relieved. While you may not be noting excessive drag at the rotor (the ATE will likely have a little under even the best circumstances), this may explain why the lever free travel is not what you expect.

You could try to flush the brake fluid and see if that helps, but you will likely be amazed at the difference new seals (and a piston, since it comes with the kit) will make.

Good luck!:beer

PS: Lew is right- pics would help!
 
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Jad01

I think I will just rebuild the calipers and get back to the thread with results. It's the only thing that make sense.
When I first adjusted the calipers/pads, I used a feeler gauge to get the front/back position the same. However, the wheels would not release after applying the brake. (That should have been a clue) Having not done this before, I wasn't sure what to expect. Then after checking everything else, I adjusted the calipers/pads so the wheel was free, disregarding the parallel requirement. That freed the wheel, but didn't really affect the free play. More to come. Again, thanks to all for the input.
 
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