Excessive brake lever travel

[Barry]

I have an odd problem which started with some unusual but persistent brake squeal from the front disk. I removed the pads for a quick deglaze and put them back. I now have more travel at the handlebar lever than before and It can't be air in the system as I didn't disturb the hydraulics and in any case  tying the lever bake overnight didn't make any difference.

We  know how the caliper seals are supposed to retract the pistons a few thou from the disk but when I remove the caliper dust cover and observe the pads while operating the brake one pad is retracting far more than I've ever seen before and significantly more then the other. I believe that is the cause of the additional brake lever travel.

As I said it's an odd problem and I can't think what the cause might be. It's much more usual for pistons to not retract enough and for that to lead to glazed pads and brake squeal.

The brakes work OK I just don't have the rock solid lever that I'm used to. Anybody have an insight ?

[nhmaf]

It might possibly be some dirt or crud either partially blocking a hole in the caliper, or a buildup of accumulated dirt around the outside of the piston that is preventing it from retracting back into the caliper.   Or, it could be neighter of these - but they are the more common causes of either brake dragging or pisotn not fully retracting.   Dod you look at the pistons when you last had the pads out for cleaning?

[Barry]

The problem I have is that one piston is retracting too much which is why I described it as an odd problem to have. I'm struggling to think of a mechanism that would cause too much retraction as opposed to not enough which is the more usual scenario. Maybe a vacuum above the fluid level in the master cylinder ?  Where exactly is the hole that lets the master cylinder reservoir breath ?

[nhmaf]

I am speculating that the reason you have more lever travel is because one of the pistons is extended -> thereby  requiring displacement of more brake fluid, which perhaps might cause the other piston to retract further - the chambers are connected fluidically, so perhaps if the one piston will not travel back, it is causing the other piston to travel back further, as a certain volume of fluid must be displaced from one piston to the other.   IF both pistons are free to move, they will each move a smaller, roughly equal distance.

There are, to my knowledge, only (2) intentional holes within the master cylinder reservoirer - both on the bottom of the chamber.  The tiny, needle-point sized one is the one that most often gets gummed up and serves as the fluid return circuit.   But, if that were plugged on your bike I'd expect that both pistons wouldn't be retracting properly.   But, it bears looking into, I think.

[Mike V]

Just a shot in the dark here,

1) I would try another system bleed making sure no air remains in the system keeping a close eye on the return port making sure no bubles or foam appears.
2) damaged or rolled square o-ring?

Interesting condition, curious to hear what you find...

[Barry]

Problem solved - pretty much anyway.

The diagnosis.

I removed the master cylinder lid and operated the brake to observe the usual little squirt of fluid No difference in lever travel with the lid off so buttoned that back up again.

Had a look at the pad movement again and as before the left pad was clearly retracting more than the right. This time though with the aid of my best reading glasses I could see why. It was the disc that was pushing the left pad back !   When I operated the brake lever and released it the disc itself moved to the right and back again. Presumably the disc was actually bending. How could that be with a twin piston caliper surely it should centralise ?  When I looked at the pads they had not been contacting the disc over the whole surface area. I must have put them back in the wrong relative positions even though I thought I'd taken care to put the left pad back on the left and the right on the right (I trusted to memory so we'll put that down to me getting old).

Why should it matter which way the pads are put back ? I can only assume the pads have worn very sightly tapered because of less than perfect alignment of the caliper with the disc.

With the pads swapped over the problem is all but gone and the lever travel much reduced. I believe I can still see a very slight movement of the disc but it's much less than before. I wonder if that is why floating discs are supposed to be the things to have. I think I'll have a very close look at the caliper alignment and see if some fine shimming is in order. Perhaps if I put the digital calipers on the pads I'll be able to measure the tapered wear.

If anyone gets chance flip off the plastic cover and see if you can see any movement at all (a couple of thou at most) of the disc while operating the brake.

[Mike V]

Barry,

Well done.  "Simple First".  My guess would be if you're seeing substantial flex in your rotor during brake application after reassembly of the pads you may need to check the size and position of the caliper offset sleeves.  There was a thread on the airlist recently with a quick and slick measurement to determine size of offset sleeves needed.  I can search and post later when I get home from work, if you like. I believe the Real OEM site will list the different sized offset sleeves. See #14 in the link provided...

http://www.realoem.com/bmw/showparts.do?model=0364&mospid=51897&btnr=34_0595&hg=34&fg=11

As a footnote, I can see a very little bit of flex in my rotor(s), very slight - which would point towards caliper centering index.  But, for me it's not enough to consider a different fitment or adjustment in my existing offset.  Love my front bakes on the 81 R65.

[Barry]

Thanks for the observation Mike.

The very slight flex I now have sounds like it may be normal enough.

There is no offset adjustment of any sort on my ATE calipers. I was thinking I'll make some thin shims if it proves necessary but for now I'll let the pads bed in again.

[saintell]

As you have found the remedy this tip might not be any help to you but when taking out pads and putting in new ones it is a good idea to loosen the big caliper bolts and push the bike forward by sitting on the seat and at the same time apply the front brake hard,do this a few times then tighten up the big caliper bolts,this procedure will align the caliper to the rotor,cheers Nev

[Ed Miller]

As you have found the remedy this tip might not be any help to you but when taking out pads and putting in new ones it is a good idea to loosen the big caliper bolts and push the bike forward by sitting on the seat and at the same time apply the front brake hard,do this a few times then tighten up the big caliper bolts,this procedure will align the caliper to the rotor,cheers Nev

Do you keep a hand on the brake lever whilst snugging up those caliper bolts?  I've never heard of that trick, and it sounds like a good idea.

[saintell]

Hi Ed,sorry hadn't been reading for replies,yes to answer your question,the idea is to let the caliper find it's own place and tighten up the bolts with the handbrake still on,cheers Nev