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Thread: /7 brake woes -- frozen front brake

  1. #1
    Rally Rat nhlkats's Avatar
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    Lightbulb /7 brake woes -- frozen front brake

    Decided to start prepping the bike for riding season after over 100" of snow this winter.
    Bleeding the brakes (single disc front with under-the-tank frame-mounted master cylinder). For some reason, the brake lever remains soft, no resistance. Also, it feels like the front brake is on, b/c the front wheel does not spin freely, even though the brake lever is not pulled in. Known minor master brake cylinder leak.
    Upon closer examination, pulling the brake lever in does not move the brake pads. They are frozen stuck, onto the rotor. That would explain the lack of free spinning front wheel.
    What am I most likely dealing with here? I searched the forums for keywords "frozen", "brake", "stuck", "caliper" in several combinations and yielded unrelated results.
    A push in the right direction would be obliged.
    Thank you.

  2. #2
    Liaison 20774's Avatar
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    A few thoughts:

    - blocked return hole in the master cylinder piston
    - torn o-rings on the master cylinder piston
    - internally damaged OEM rubber hose (any bulges along the line when you operated the lever?)
    - corrosion inside the caliper not letting the piston move either way

    What do you mean by "known minor master brake cylinder leak"? If you know there's a leak, you really should fix that first.

    If the o-rings are torn on the master cylinder piston, then when you squeeze the lever, the fluid just simply goes back and forth past the o-rings instead of being shoved down to the caliper. That could be the source of the softness. Or you could still have air in the lines.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    I take it the front wheel is not completely locked? First, pull the caliper and make sure something else isn't causing the friction on the wheel. I would keep open the possibility that there are two problems: one, air in the hydraulic system and two, a frozen caliper. Use a C clamp to push the piston back into the bore, then rebleed. If you still have no resistance at the lever, then something is wrong with the hydraulic system as you should get resistance with a frozen piston. Then you're looking at a rebuild.
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '14 Street Triple R, '17 1290 GT (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  4. #4
    Rally Rat
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    It's time

    Time to rebuild the brake system. Rebuild or replace the caliper, master cylinder, and replace the rubber brake lines. Anything less just won't be good enough. Those brakes are old. They lasted a long time and have served you well. Put them out to pasture. Good luck.

    Ride Safe
    Rideoften

  5. #5
    Registered User 37071's Avatar
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    O-ring

    When I was changing the brake fluid in my R90/6, I found that the o-ring between the brake fluid reservoir and the master cylindar would no long keep the brake fluid/paint stripper in. This let air into the brake line. After replacing that o-ring and bleeding the system everything worked again.
    Does releasing the brake lever not suck the pucks off the rotor - as long as there is no air?
    Gar

  6. #6
    What causes the caliper pistons to retract is the resiliency of the square cut rubber piston seal.

  7. #7
    Don't forget your towel
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    There is a veritable host of possible problems here. My guess is they fall into two camps; one storage, the other age related.

    Storage: A sad thing to have to do to a bike (or any motor vehicle) they jusy don't respond well to prolonged dormant spells in the cold, unheated garages where most of us keep them.

    Corrosion build-up on the caliper pistons is a classic cause of "frozen brakes" there isn't much drawing the pistons away from the disc other than the above mentioned tension from the seals and whatever lateral play there is in the disc itself. The solution is pull the caliper, clean it and replace anything suspect.

    The pads themselves might also be stuck in the caliper, cleaning should solve that too.

    Age related: What can I say? We all get old. Fortunately it's easier to replace bike parts than people parts. That soft brake lever could be any number of things from hoses to seals gone bad. Squeeze the brake lever while watching the hoses, do they swell? They should not. I was trying to figure out a soft pedal on my F650 and had a hose rupture while up on the lift, that bike was built in 2001.

    You mentioned a slow leak, is it faster now? Is there a small jet of brake fluid shooting across the room? It's been a long time since I had to rebuild an under-tank master cylinder, assuming the kits are still available and your MC isn't shot from corrosion a rebuild isn't that hard.
    Steve
    "...your body is not a temple, it's an amusement park. Enjoy the ride" A. Bourdain

  8. #8
    Rally Rat nhlkats's Avatar
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    All,
    Thank you for the responses.
    The "minor" leak of the master brake cylinder was noted at the end of last riding season, a rebuild kit is on its way. It seeps a very small bit of green fluid from the piston area when the piston is moving in and out. It has leaked onto the frame. It is not enthusiastically seeping or hemorrhaging.
    No bulging of the old rubber lines was noted. However, they are cracked and old and should be replaced.
    The front wheel is not completely locked. It can rotate with little effort, but it will only spin a little bit then stop. When dead rolling the bike, you hear the squeak from the pads being locked onto the rotor, and feel the slight resistance from that.
    After bleeding the brakes until I saw no air bubbles coming out, the brake lever did build a little bit of resistance. That is, it is no longer completely limp soft, and the lever can no longer be pulled in all the way, 100%. However, it is far from the stiffness that it was last riding season. If I dead roll the bike, I can bring it to a stop with the brake lever, although it must be pulled as far in as possible, whereas "normally" it would only take about an inch in to stop the bike. Again, I don't see the brake pads move, but there is braking taking place when the lever is pulled in as far as possible, although it is weak and can be overpowered by simply pushing my weight back and forth onto the bike. Hope that makes sense.
    The bike has sat through the rough Wisconsin winter outside, covered.
    I will check the things you mentioned and report back.
    Thanks again.
    Last edited by nhlkats; 03-30-2008 at 05:39 PM.

  9. #9
    Airhead krehmkej's Avatar
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    What you describe sounds about like my /7 was. If your caliper piston isn't corroded too badly, it can be reused, saving $$$. BMW Hucky in Florida sells a basic caliper rbuild kit (less piston).
    Last edited by krehmkej; 04-21-2008 at 09:41 PM. Reason: spelling
    -jwk-

    1978 R80/7

  10. #10
    Grammarian no, Rider yes ISAMEMON's Avatar
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    Rebuild master cylinder, rebuild caliper, replace hose

    like said, Huckys sells a minor kit for about half or less then a BMW caliper kit, and replace the hose with a stainlesss hose ( Speigler or other)

    got the extra bucks, replace the front pads with EPE pads

    do all that as listed above, and the bike will stop better then new

  11. #11
    On my 76 R75/6, I rebuilt the caliper and replaced the old brake hose with a new BMW rubber hose, but results were not satisfactory. I recently cleaned and lubed both of the ends of the cable from the lever to the under tank mc, and replaced the new hose with steel braided hose, and the difference is remarkable. They are as good as they are going to get, and I can trust them now, where I couldn't before.

  12. #12
    Grammarian no, Rider yes ISAMEMON's Avatar
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    two more things to add
    1) dont forget to lube the pivot bolt for the caliper
    2) these bikes are not just like some modern stuff where you throw pads in and go, you have to adjust them, there are tircks to it, check the web , clymers, etc, and make sure your pads are making good equal contact

  13. #13
    Rally Rat nhlkats's Avatar
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    Quote Originally Posted by 37071 View Post
    Does releasing the brake lever not suck the pucks off the rotor - as long as there is no air?
    Correct -- releasing the brake lever does not pull the pads off the rotor.
    The brake lever has about 1/2 the resistance it should have. Front wheel does not spin freely.
    I gave the caliper a gentle whack with a rubber mallet, and immediately the front wheel was able to spin freely without resistance. Pulled the brake lever in, saw the pads contract and stopped the spinning wheel. The brake lever had little resistance and pulled all the way in. Let the brake lever out again. Pads did not retract from rotor. Front wheel does not spin freely. Pumping brake lever now -- it has returned to about 1/2 resistance it should "normally" have.
    Will be disassembling caliper shortly.

  14. #14
    On the Road
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    Quote Originally Posted by nhlkats View Post
    Correct -- releasing the brake lever does not pull the pads off the rotor.
    The brake lever has about 1/2 the resistance it should have. Front wheel does not spin freely.
    I gave the caliper a gentle whack with a rubber mallet, and immediately the front wheel was able to spin freely without resistance. Pulled the brake lever in, saw the pads contract and stopped the spinning wheel. The brake lever had little resistance and pulled all the way in. Let the brake lever out again. Pads did not retract from rotor. Front wheel does not spin freely. Pumping brake lever now -- it has returned to about 1/2 resistance it should "normally" have.
    Will be disassembling caliper shortly.
    Did you use synthetic or natural fluid? I learned the hard way, my /7 and synthetic brake fluid do not mix. If you totally purged the brake line, it's going to take awhile to get the air out. My method is to pull the break lever in while the valve is open (on the caliper) then close the valve and let the leaver out...keep doing this until you get adequate pressure on the lever (does not feel spongy). I let the bike rest for 24 hours and do it again.

  15. #15
    Rally Rat nhlkats's Avatar
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    Crap...

    Please advise, how the hell do you get the "primary and secondary seals/cups" onto the master brake cylinder piston without hurting it or yourself? It ripped and flew across the garage on unsuccessful mount attempt #4. Hopefully I can order them separately.....

    Yes, I was using synthetic brake fluid.

    Thanks.

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