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The rear disc brake on the Jitsie Varial 24" Hybrid that I recently got (for Christmas, so about two weeks ago) started squealing and losing its power. This was also a problem on the hardtail mtb that I was riding on before.

I've cleaned the rotors with isopropyl alcohol, sanded down the brake pads, and bedded them in by riding around my neighborhood and slightly pulling the brake lever. This would fixed the squealing noise and loss of power for about a day then it would come back. When that didn't work, I tried replacing the brake pads. That would fix it for slightly longer, but it would still come back after maybe a week.

I have tried to be very careful about not contaminating the pads. I haven't washed my bike with anything yet, and I wear rubber gloves when touching the rotor. Before riding trials I never had any issues with squealing brakes. Is it normal for that to happen continually and after such a short period of time? If not what could be the problem? I did notice that both bikes were running Shimano B01S Resin Brake Pads. I'm not sure if that could be a problem.

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    Have you done 20~30 hard braking efforts from "going pretty fast" to completely stoped ? That is a better bedding-in process than riding around the neighbourhood with the brakes dragging lightly. Once the pads are glazed their braking maximum is less. – Criggie Jan 8 at 2:51
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A couple things to look at:

  • Perhaps try Brake Clean to clean the rotor and spray on a rag then wipe the pad surface. I successfully use it on rim brakes when they occasionally squeal and it is not due to the toe angle of the pads. However be careful with it as it is stronger than rubbing alcohol so do not get it all over the place.
  • It could be possible the rotor is out of true. In which case, you should go the shop you got the bike at to get that addressed.
  • You could possibly have a bubble in the brake line which is causing a lack of pressure to close the caliper and that in turn is causing vibrations as you slow along with the loss of braking power. Again the shop that you got the bike at should be able to address that for you pretty quickly.

Hope that helps.

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  • What is a good brand of Brake Cleaner? When initially got the bike I took it to my local shop to get it set up properly (I got the bike online). Would they have fixed the last two problems if they were occuring? – Max Marcus Jan 8 at 22:45
  • @MaxMarcus - CRC BrakLeen - amzn.to/38toGLP . I use this stuff on rim brakes, car brakes, and as a general degreaser. Just be careful with it as in do not get it on stuff you do not want to clean and I try to avoid it touching paint or rubber or if it does I wash it off right away. Auto stores should have it on the shelf in the US. – Tude Productions Jan 9 at 0:48
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Have you ever listened to what a racket the rim brakes on comp trials bikes make?

This is by design: comp riders roughen up the rims so the brakes will lock up better. Deceleration, what every other discipline uses the brakes for, is hardly relevant for them.

Now, I actually have the Varial Hybrid myself (since November), and I too was surprised by how squealy the brakes were to start with, but I assumed it was also intentional (though I couldn't really think how they would have done this with the disc brakes).

By now the brakes sound just like the ones on my hardtail MTB (which makes sense – also Shimano 2-piston with resin pads), and they brake similarly too. Unfortunaly they also don't lock up as promptly anymore, but if you're more into street-style riding that's probably fine for you, so just be a bit patient.

Of course it's possible that your brakes really have an unrelated issue that my Jitsie doesn't have, but before you worry too much give it some time first. You're (hopefully) not going to send big downhill tracks on that bike, so if the deceleration isn't optimal you should still be ok.

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  • After riding on for the past day, it has started rolling for a few inches after doing a pedal kick or gap. Would that be expected? – Max Marcus Jan 8 at 22:41
  • I'm not sure what you mean... it doesn't lock promptly enough? – leftaroundabout Jan 8 at 23:10
  • Yes... Initially when I would land after a gap I would immediately stop on my back wheel. Now when I land a gap the rear brake loses its grip and the wheel rolls a couple inches, making me less precise and sometimes lose my balance and have to step off the back of the bike. – Max Marcus Jan 8 at 23:54
  • It doesn't do that for me – but then, I can't really do more than small backhops, so I'm probably just not putting as much torque on that brake as you do at your higher skill level. Yeah, as I said, the brakes definitely need quite some lever force to actually lock up. — I think the stock Shimano brakes on that relatively inexpensive bike just can't be expected to be a match to professional trials brakes. I've thought about upgrading the rear one to a Magura MT7, but so far the Shimanos are enough for me. – leftaroundabout Jan 9 at 0:25
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It sounds as if you are glazing your brake pads.

Pads usually make noise when they are brand new, contaminated or glazed. If you do not break in your pads adequately, there is a great chance they will become glazed. This can be caused by braking hard with new-ish pads. The heat will polish the surface and greatly reduce the braking power, thus requiring you to apply more power on the levers and heat building up.

Non mineral oil has a tendency to boil when heated up, i.e if you brake hard or if your brakes heat up too quickly. This results in the lever becoming mushy and the braking power reducing significantly until things cool down again.

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