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GSXR-750 K9 Rear Brake Overheated

Discussion in 'Technical and Troubleshooting Torque' started by assalible, Oct 2, 2016.

  1. Hi all,

    Need a little assistance solving this problem :)

    So bought a bike a week ago. Rear brake didn't work. Got me a brake bleeding kit and some liquid and managed to get the brake to work properly. Fast forward one week of riding until today. Had some fun in the mountains and result is as shown in the pictures. Any ideas why this happened? My theory is the piston being stuck, brake was applied all the time and during a longer run the liquid heated and expanded thus forcing the piston out.

    The brake was bled (cheap brake liquid) one week ago and was working fine.
    I didn't ride the bike more than 10km at once during one week.
    Had a longer ride today and brake overheated seriously (see pictures)
    As far as I remember I did not use the rear brake while riding (haven't used it for 10 years). And as I did not adjust the pedal, my foot seemed to be on the side of the lever or even under it making it hard to accidentally apply the brake.
    Possibly tested the brake before the ride today or yesterday to see if repair was still holding (not riding).
    Previous owner also claimed not to have used the rear brake at all.
    I did not feel the brake to be applied while riding (in the mountains) and when we stopped, it was not hot any more and all the damage was already done.


  2. Jesus, that is farked! There must have been some serious heat going through that to melt the piston and warp the pad. You never felt anything weird??

    It doesn't look as though the piston itself has been cleaned for a while, so it'd probably be fair to say the maintenance has been slack. Forget the rear i'd be checking the fronts, pronto.

    You've got Goodrich braided lines, good. Bin the caliper, buy a second hand unit, recondition it, stick some pads in and go again. You don't use the rear brake so a second hand rotor or fancy Chinese wave one will do if cash is tight.

    oldcorollasoldcorollas knows more about twisted metal than most.
  3. Yup, didn't notice anything. When we stopped for a moment i briefly felt a strange smell but it went away and I didn't manage to check the bike. Of course feeling a difference in handling was a bit hindered as it's a new bike and these were new roads for me (trying to keep up, plus its gas on/brake on all the time in the mountains). Even went trough some gravel, so certainly no lock-up.

    The bike itself was maintained very well, even I can not match that standard. The piston might be dirty because there was a leak of break fluid and a lot of pad and piston material around there. I examined the piston myself before plus it went through roadworthy.

    Probably will go for a cheap rotor as it's there only for a show anyways. I am not that good to actually need the rear brake.

  4. I have had a rear brake drag after I adjusted to little free play and over heat,I soon found it as it slowed the bike noticeably,that's a bike with 40 to 50 hp,not 100 or so like yours. But flippen hell thats a lot of damage,you couldn't smell it.
  5. True.
    It'd be interesting to hear others' views on this. I've never seen anything like it.
  6. Be extremely careful of that disc,all that heat would not do it any good.Replace it,shattered discs kill people.
  7. I have heard of wearing pads out in a day with foot resting on a pedal but not that
  8. The disc it not straight anymore so a new one is required. And as I explained, as far as I know, it would have been hard to have the foot resting on the pedal..
  9. that's proper fcuked! good job there!

    i'd be replacing the caliper mount too.

    aluminium casting alloys for brake/automoative bits are often stuff like 354, 355, 357 etc etc,
    typically fully liquid at >600deg (liquidus temp) or so, but start to get mushy at ~550deg (solidus).

    the piston has had pressure on it while above 550 (but less than 600), as the stuff pushed away has a mushy semi-solid look about it.
    the splashy looking stuff on the outer looks to have been proper liquid (looks like dropped solder).
    wonder if that is the friction material or the caliper? :p

    and yeah that's bad... but is that backing plate bent??? steel also starts getting petty weak around 600+,
    the pads would have been hotter than the piston was, and backing pads somewhere in between..

    random schematic of strength vs temp

    anyway, with all that heat so close to the caliper mount/bracket, there is a chance it has been heat affected too.
    it is a cast alloy, but if it has any heat treatment or precipitation hardening, the bracket may now be overaged, and compromised in strength.

    Hopefully the caliper bracket has shielded the swing arm from the radiant heat enough that it is not affected.
    would still have a look and see if the swingarm powdercoating has burned off or been affected.

    as for cause, only a slight touch is enough to generate heat. doesn't even need to be enough to slow you down. stuck or sticky piston, pushed out once and slightly dragging..
    the disk would have been getting up into visible red hot territory, and once it reached there, the damage to the aluminium could have been done very quickly, as in less than a minute (even 10's of seconds), but more likely to have been a slow continuous expansion as you suggested

    not the required ~600+ is getting into visibly red territory :)
  10. oh, and i'd be looking seriously at the hub where the brake disk bolts to it.
    any sign of heat affecting it (as in melting or mushyness).. and should probably be considered toast too

    Metalgear disks are reasonably cheap and seem to be half decent.
  11. Wow, that was a nice answer :)

    Yes, the pad backing plates were also bent.
    I'll consider the warning about the bracket. But as the brake is only for "cosmetic" purposes, probably it will stay. Addionally, had to order replacement stuff through mechanic to actually get RWC done and bike registered within time limits.
    I'll disassemble it later to check for any signs of heat treatment.
    As I was trailing on the ride then probably everybody missed the nice glow effect tuning :( I'll try to mount camera for the next time!

    One more question. As I really found it hard to explain to have my foot on the pedal for that time, what should I do to avoid future mishaps. Can not remove the pedal.. Any suggestions? Trimming it as high as possible seems only solution.

  12. i think you may have used it once, and the piston stuck out and started dragging without you putting foot on pedal any further

    err.. you could increase strength of the pedal return spring? then it is still there if you need it, but harder to accidentally drag... (not sure how you would tho)
  13. Are you trying to prevent 'dragging' the brake by inadvertently resting your foot on the brake lever?
    Do you ride with the balls of your feet on the pegs, or do you 'hook' the heel of your boot on the peg?
  14. Hi all,

    Unbelievable, but it happened again today on the freeway (no braking for at least 10 km). Fortunately, this time I could easily feel it and managed to pull over. By that time the rear was almost locked. Brake pedal was rock solid and it was very hard to move the bike (did it just in case to avoid disc warping). I let the thing cool down and when I wanted to remove the brake just in case, it was already working properly again. Ride slowly back home, checking the brake often and it did not even get too warm and worked fine (tested several times).

    Is the master cylinder getting stuck and not releasing the pressure? I am guessing, if there is too much pressure the liquid should flow to the reservoir (it is not overfilled)? I already replaced all the other parts (disc, caliper, caliper bracket).

    Seems I missed the previous replies. I am riding mostly on the heels. For some faster riding going on the balls of the feet. I have been checking my foot position and its far away from the brake..

    It's getting mysterious for me. Maybe should go to a psychologist to check if my right leg has a life of it's own. And I feel like developing a disc touching fetish (always checking if it's hot). :emoji_grin:


  15. pressure would only be relieved if the master cylinder piston moved back far enough to expose the hole (which connects to the res)

    pull the master.. but first, apply a few times, while cold, see if it locks up.

    and dumb question... is the rear wheel pointing straight?
  16. not sure how your rearsets/master cyclinder are setup, but if possible you could try ones of these, i need to fit this to my bike too. since fitting my new rearsets i'm a bit worried i might have the same problems in the near future.
    Graves Motorsports Sport Bike Rear Brake Return Spring
  17. #17 German mistress, Oct 16, 2016
    Last edited: Oct 16, 2016
    When I installed phil Taiton rear sets on my 2010 gsxr 750 it came with a long Spring ( see it at the back of the rear set in picture .)

    I never had a problem with the stock rearsets with zero Springs.

  18. Check/clean the relief port in the master cylinder.

    I experienced these symptoms when i first bought my ZR7. On multiple occasions the front brakes just grabbed and held tight on my commute to work. Lever would go hard as a rock. Turned out i had the tiniest bit of gunk blocking the relief port in the master cylinder. The master became a one way valve: fluid in, but not out. Pressure would build, but not release.

    The relief port is a very small orifice. You'll need a fine copper (soft) wire poke it clean.

    • Like Like x 1
  19. Check that the brake lever itself is fully returning (weak spring, lack of lube) that will turn the master cylinder into a one way valve.

    Ask me how I know........
    • Like Like x 1
  20. Brake is working fine while cold and also most of the time while riding (have tested numerous times). And it doesn't get overly warm.

    Rear wheel seems straight and the markings on the swingarm show that too.

    Brake lever spring seems properly strong and it returns nicely. As far as I understand, the master cylinder has another spring to push back the piston. If the piston is stuck, the lever still returns to zero position. Am I correct?


    PedalSpring.JPG MasterCyl.JPG