Rear brake locks up K1100LT

[BlueK11LT]

I recentley replaced the rear pads on my K1100LT and adjusted the set screw and locknut on the pedal so that the pads would start to bite with minimal downward movement of the pedal and the wheel still be rotating freely without the pads binding.
All was well for over 500 miles until yesterday when I engaged in a bit of 'sportier' riding involving my heavier than normal use of the brakes rather than my usual slower,laid back style of riding.
The bike suddenly started to feel sluggish and slowed right down to a stop as i pulled into the side of the road.
There was smoke coming from the rear disc which had locked solid.
I got out my toolkit and loosened the bleed nipple to release a small amount of fluid and the brake freed up immeadiatley
To be on the safe side iam replacing the rear rubber hose with a stainless steel braided hose in case it is collapsing internally and possibly has became prone to working like a one way valve.
I have now backed the adjuster screw off again.
Iam also wondering if when I adjusted the rear pedal screw the pads have still been in just enough contact with the disc
to quickly build up heat when braking heavily and so heat and expand the brake fluid forcing the pistons to push against the pads?
Any input appreciated.

[alexis291]

I had exactly the same problem on my 1100RS. I thought I had an engine problem but when I stopped my calliper was on fire. Luckily I managed to blow it out before the whole thing went up. Like you I didn't want any play in the linkage but ended up with none. I now run the back brake with some slack to be on the safe side.


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[mr_haut]

I'm wondering if it would be relevant to replace the brake fluid.
If you adjust for minimal/no slack, then water in the fluid would under "sportier" conditions turn into vapour, giving you the "automatic-slowdown-feature".
Changing the line for a steel braided on e, I assume will include a full bleeding of the brakes anyway.
Personally I prefer a slight travel on the rear brake, before it engages.

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[Laitch]

Any input appreciated.

When was the last time you replace the brake fluid and inspected the caliper pistons?

My experience is that overheating brake fluid causes brake fade, not seizure. Overheated pads might expand enough to seize the rotors. Collapsing or clogged hoses could contribute to retraction failure. Dirty caliper piston bores and worn seals could contribute to retraction failure. A failing, dirty master cylinder might cause retraction problems. Your bike's brake system is due for a thorough inspection.

The attached diagram shows the correct freeplay spec for adjusting the brake pedal. This is from the BMW K1100 service manual available in the Repair Guidance section's Master List here on the site. It also contains other adjustment specs necessary for the safe and efficient operation of your bike.

K1100 Brake pedal adjustment, from the BMW Workshop Manual in the Motobrick.com Repair Guidance Master List.jpg (49.98 kB . 510x576 - viewed 944 times)

[Martin]

As per Laitch but you definitely need free play. When anything heats up it expands with no free play pads probably engaged the rotors.
Regards Martin.

[BlueK11LT]

Thanks for the replies - I had seen that diagram previously Laitch and the screw I adjusted after fitting the new pads recently was the one which is numbered 3 in the diagram and I also bled out enough fluid so that all the old fluid was expelled and checked that the caliper pistons were moving freely.
I do happen to have a braided brake hose previously fitted to another bike of approx. the correct length which would be perfect for the BM except that the the female union at one end has m10x1.25 threads whereas the bike uses m10x1.00 threads on the male union which is on the steel pipe  between the rear hose and the abs modulator.
I guess I could take the steel pipe to my local car parts store and get the male union replaced with one of the correct thread pitch and save myself the cost of a new braided hose.

[lmiklosy]

... replaced the rear pads on my K1100LT and adjusted the set screw and locknut on the pedal so that the pads would start to bite with minimal downward movement of the pedal...

Laitch and Martin already addressed free-play, new parts will remove free-gaps between them after reassembly. Tightening the adjusters to "bite" with minimal pedal movement was your undoing.  Due to wear and corrosion the tolerance stack-up between parts can change after reassembly.

This just happened to me on a 36 year old 1981 R100RS.  I thoroughly cleaned the front brake system including the master cylinder and the front brake seemed normal,  but on a test ride it bound-up on the San Diego 405 freeway.  Dismantled the master cylinder again to find the MC piston was covering the single supply and return passages in the master cylinder.  Taken a step further, here's a photo of the front brake master cylinder showing the supply and return holes. On this particular cylinder the supply hole (left) is clear, the return hole(right)  was never drilled through. Yes that is correct, perhaps a quality problem.  The blocked passage only arose because I took the cylinder apart to clean it, when reassembled the parts fit tighter (looser?) covering the supply passage.  At freeway speed with a hot motor cooking the brake lines and hot brakes the brake pressure elevated, with nowhere for the pressure to go the brakes applied. 

My K75 has a similar Magura master cylinder with a supply and return hole, the return hole appears to have a ceramic or fibrous filter covering it. So far despite my cleaning that brake system, the blockage didn't occur and no brake lock-up.

If there is a BMW service manager reading this maybe he/she can explain the missing return hole on the R100RS brake master cylinder for that model and year 1981. If any other "R" owners out there have experienced this problem please share your story.

[mystic red]

Yes, follow the rear brake adjustment directions in the manual to the letter or have a fire extinguisher handy.