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Changed tyre now rear brakes are always on

Discussion in 'Technical and Troubleshooting Torque' started by Salad Cycle, Jul 14, 2015.

  1. Hello,

    I changed the rear tyre on my cbr125r and now the rear brake pad is always in contact with the disc. This slows the bike down significantly and will no doubt use more petrol. How can I free up the back brake?
  2. It will also heat up the brake fluid, increasing pressure in the system, to the extent that the wheel may eventually just lock up on its own accord. If the brake is stuck on, it's not safe to ride.

    The question is, why has the brake seized? The first thing I'd do is double check that I'd installed the caliper and pads correctly. Then I'd pull the caliper off any try to work out what in the hell is going on with the piston (i.e. Can it be moved?).
  3. Was there a spacer in the rear assembly? Is it in the correct side? Is the rear wheel aligned correctly? Don't trust the marks on the swingarm. How's the chain tension?

    Alternatively you may have pushed the piston too far out of the caliper and it's not going back in. Pull the caliper and take the pad out. Try to push the piston back up (remove the resevoir cap).
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  4. using more petrol is the very least of your worries :)
    • Agree Agree x 2
  5. As ibast said, I would look at any spacer and ensure that the rear wheel seems centred in the swingarm, and that the adjusters on each side are set evenly so that the disc is running parallel, but if that were the case, I would suspect you couldn't have got the caliper back on, well not easily anyway.
    Worrying about it using more fuel, that surely has to be an Asian thing??? It's the least of your potential problems!
  6. Do be aware that most brake pads don't actually lift off the disks, they just float over the surface. But you did say it slows the bike significantly.

    Before disassembling anything assuming it's a single piston calliper then the calliper slides on pins so that non piston side pad squeezes the disk too. As you squeeze the brake pedal, the pad moves in and the calliper body moves the other way, pulling the other pad with it. you should be able to just push the calliper towards the disk with your hand, which pushes the piston back in to the calliper. but it's not a lot of movement. You will probably need to use the brake pedal to push it out again.

    If it is stuck on, first problem will be heating up and wearing out the disk, possibly even glaze the disk, sort of like polishing it and then you loose most of the friction.

    Do make sure you keep the friction surfaces clean. Absolutley no grease or oilm on them. Acetone or meths and a clean cloth if you've nothing else.
  7. There are two wheel spacers of different thickness and diameter. They only fit in the wheel in one order due to their size so I believe they are in correct order.

    Chain tension is near the 3cm recommended in the manual.

    I noticed the reservoir cap was off which may have let air into the brake lines however the nut was secure.

    Just joking about using more petrol.

    I was fiddling with it a bit more and noticed the brake lever no longer moves the caliper so I have fiddled a bit too much.

    I have decided to get new brake pads, brake fluid and disassemble/clean the caliper to ensure it is working properly. I'll do this later this week.
  8. reads like you've pushed the pistons out of the caliper.

    How many ks has the bike done? Might be worth putting a kit in the caliper whilst you are there.
  9. Look on the bright side at least you don't have to work upside down in the foot well in the dark under the dash of a car trying to undo nuts on the master cylinder that haven't moved in 30 years.
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  10. 2007 model with 22905km, bought it at 18500km. I was excepting to have to replace a few things but thankfully I bought a cbr125r so it won't cost much.
  11. I have discovered that the inner brake pad is dragging on the brake disc. To confirm this I clamped the brakes open and the inner break pad was still in contact and dragging on the disc when I tried pushing the bike. The break pads appear to have plenty of meat on them. I confirmed the wheel spacers can only be in one order due to diameter.
    2015-07-15 16.04.31.
    2015-07-15 16.05.56. 2015-07-15 16.06.05. 2015-07-15 16.07.02. 2015-07-15 16.09.47.
  12. That piston is way out. It should be only abour 1/3 across that space, at the most.

    If it was that far across when it was on the bike, something was assembled in the wrong order.
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  13. Take the top off the master cylinder reservoir and using a syringe or something else remove some brake fluid...not all of it just enough so that when you compress the piston back into the calliper the master cylinder doesn't over flow. Reinstall the caliper and try again. If this doesn't work I would suggest pulling apart the caliper and then giving the piston and the bore and light sand with wet and dry. Reassemble make sure you bleed the brakes correctly to remove any air.

    I'm not a mechanic and I'm certain there are much much more knowledgable people in here than me.
  14. if that piston has popped, there is already air in there. I'd be amazed if it wasn't out of the seal there
    • Agree Agree x 1
  15. The wheel was misaligned.

    To fix this I tightened the chain nuts with the axle nut loose. Ensuring the axle was in a position that did not result in excessive brake pad friction on the discs. I went for a ride and the brake dragging is back to normal amount.

    I also noticed the bike steering to the right and corrected that disturbing misalignment.

    I pumped the cylinder out to see if it was damaged I can compress the cylinder with my thumbs.
  16. U beat me too it. I was about to say that I bet your rear wheel was very misaligned, causing the disc to be way too close to one of the pads. In the picture u can see on the bottom pad that it hasn't been making even contact with th disc. It must have been very misaligned How did u not notice that haha?

    As for the other replies, if the piston was popped out I doubt you would have had enough room to slide the disc back in but I could be wrong on that.
    • Winner Winner x 1