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Thread: Sticking front brake... master cylinder or caliper?

  1. #1
    look out!!! Visian's Avatar
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    Sticking front brake... master cylinder or caliper?

    I recently did some work on the front master cylinder of my 86 R80G/SPD+. Bike has 90k and I've never rebuilt the caliper or the master cylinder.

    I wasn't planning to rebuild anything because the brake was working fine. But as is usual for me, a simple job turns into a nightmare through a comedy of errors. It all began with a frozen screw holding the reservoir cap on. I was not able to remove it, so I bought some used parts on the IBMWR Marketplace... a complete master cylinder (part #23 below), piston and reservoir.

    Put it all together and everything worked fine. Well... there was a very small leak from what appeared to be the cylinder. After closer inspection, the leak was coming from the junction between the reservoir and the cylinder (part 23 below). I used some assembly sealant to fix that, re-assembled, and everything worked fine.

    Then, to make a long story short, a job involving an upgraded handlebar resulted in the need to purchase a new part #11.

    When re-assembling everything, and prior to filling with brake fluid, I mistakenly squeezed the front brake lever and the piston got stuck, hard. So I disassembled, unstuck the piston, reassembled, filled and bled the system.

    Now I am having the sticking problems. One problem is that the brake will stay on after releasing the lever. Another problem is a very sticky action when applying... squeeze/nothing... squeeze harder and something breaks free and the brake is applied.

    The cylinder *looks* fine... it is smooth and clean.

    I have two pistons (part #12) spare... one from this bike and one from a previous bike, so I am first going to try one of those. Neither looks worn, they both look the same as what I bought on IBMWR, but I am not sure what I am looking at.

    Not sure what to do if the same problem happens afterwards. A new piston and cylinder is $200.

    Any advice would be appreciated.


  2. #2
    James.A
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    If the problem arose after fitting a different part #11, I'd check for a mechanical bind in the action of the transfer of leverage from the brake lever to the master cylinder.

    Then after that, and speaking from general (not specific) experience, I'd check to make sure that all orifices from the reservoir to the cylinder are clear and open. You might want to remove the part and open it up. Then maybe a bench-bleed to clear the bore. This operation would not require an expense, apart from your time and some fresh brake fluid.

    In the automotive world, I experienced this once on a Chevy van where it turned out to be a brake hose that was deteriorating internally and would capture and hold the hydraulic pressure once it was applied.

    Lastly, it could be a caliper piston sticking in the caliper, but I would work this issue from the top down.

  3. #3
    Registered User donbmw's Avatar
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    All the above also.
    1975 R90/6, 1980 and 1982 R65, !959 TR3A Triumph Car

  4. #4
    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    Echoing the above responses, but I'd try first removing the reservoir from the master cylinder and ensuring the return port is clear. A strand of wire from a stranded wire is useful as a probe for this purpose.
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  5. #5
    look out!!! Visian's Avatar
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    Randy - where is the return port?

  6. #6
    On the Road
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    If its sticky action in the brake lever that you're referring to, I had the exact same problem after a MC rebuild, new brake lines, etc. In my case, the brake lever was not smooth, and the action was not gradual at all. When the lever was squeezed a bit it was unresponsive, and then suddenly it would "break free" (as you described)...this resulted in pretty jerky braking. The solution ended up being incredibly simple:

    I removed the lever (#2) from the housing (#11), and applied Bel Ray Assembly Lube to the flat area surrounding the bolt hole (top and bottom) on the lever...Reassembled, and have had perfectly smooth braking action ever since. I was worried that the problem was going to be much deeper than this, so it was a huge relief to have it fixed in less than an hour. Give it a try.

  7. #7
    . AntonLargiader's Avatar
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    Ian,

    How old is the brake line?

    I agree, though, that lubricating the lever has given me huge payback on many bikes.
    Anton Largiader 72724
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  8. #8
    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    As you look down at the top of the m/c with the reservoir removed, you should see 2 ports running in line along the direction of the piston stroke. One should be larger, the other smaller and closer to the outlet connection. The larger port is the supply from the reservoir to the piston, the smaller is the fluid return from the line to the reservoir. I have had it on customers' bikes that exhibit similar symptoms that I'll insert a fine wire into the return port and see a small geyser of fluid, after which the brakes return to normal operation.
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  9. #9
    . AntonLargiader's Avatar
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    Re-reading this, I'm not sure why you're messing around with someone else's used MC to replace your own used MC. At $200 (well, a bit less for me) I just buy new ones, and I'm pretty sure you have a better job than I do.. My experience is that used brake parts have a VERY short shelf life once they're uninstalled.

    If the piston is not returning, the problem is in the MC. Nowhere else. But I would also be checking the brake line for that crunchy thing... you might actually see bits of material at the banjo bolts. I had it, also, on my '88 GS. New line fixed it. At this point I have rebuilt the caliper, replaced the line and replaced the MC (12mm) and, logically, everything is great. Actually my old MC was fine but I wanted to go down to 12mm.
    Anton Largiader 72724
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  10. #10
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    prior to filling with brake fluid, I mistakenly squeezed the front brake lever and the piston got stuck, hard. So I disassembled, unstuck the piston, reassembled, filled and bled the system.

    The other possibility is that you damaged the piston cups.
    61 Gold Star, 76 R90S, 03 CBR600RR, '13 690 Duke, '14 Street Triple R

  11. #11
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    Anton - I am CHEAP! (even cheaper than you!)

    Everything was working well with the used parts until I mistakenly squeezed the brake lever with no fluid in it. I bet ccolwell is on to something.

    I'll try disassembling, cleaning and see if that works. If not, then I turn into *not cheap* and buy a new MC assembly, piston and caliper rebuild kit ($300) and that should fix the problem.

  12. #12
    . AntonLargiader's Avatar
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    A new, dry MC does not stick - in the slightest - when you fully depress the piston. Just sayin'...
    Anton Largiader 72724
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  13. #13
    ONE LESS HARLEY 04r1150rs's Avatar
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    Ian check with Motobins for MC, you will be surprised how much cheaper you can get one for..$145USD, plus shipping. unless you want to support the middle man..LOL
    Richard
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  14. #14
    2002 R1150R
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    Hey Ian,

    To test where the issue is, apply the brake, see that the wheel is stuck, loosen the caliper bleed screw a little, if there is pressure in the line, it's before the caliper. if no pressure, then it's the caliper piston. It could be a flexible hose, again, apply pressure, loosen connection - inspect for pressure. Occasionally brake hoses deteriorate from the inside, causing the pressure to be held at that point (almost acting as a one-way check valve).

    BE CAREFULL!! brake fluid will destroy paint. Make sure all painted surfaces are covered -brake fluid under pressure can spray quite far.

    Kyle

  15. #15
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    Quote Originally Posted by 04R1150RS View Post
    Ian check with Motobins for MC, you will be surprised how much cheaper you can get one for..$145USD, plus shipping. unless you want to support the middle man..LOL
    Hmmm.... I used to buy from them all the time before the dollar got so weak against the pound. Their service was always great.

    I put a braided steel line on the bike, geeze, 20 years ago. Certainly it couldn't be clogged!

    If I can't fix this with the pieces I already have, I will give them a loot.

    Thanks everyone for the advice.

    Ian

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