Brake rub help
 

Hi folks,

So I am having this rear brake rub issue on my hydraulic disc brakes. I've already tried truing the calipers by loosening the bolts and depressing the brakes and then tightening. Rub comes right back after depressing the brakes again. The disc itself is flat/true. So I am suspecting the pistons are not going back in all the way when releasing the brakes. In this case is the best approach to drain some fluid? And if so, is there a way to do it without bleeding and doing all those steps? Can I just attach a syringe and pull some fluid out without introducing air?

Thanks!

Has it always been there or just start? Braked down any long steep hills recently?

Before doing something radical like draining fluid. Have you tried folding a business car in half, loosening your caliper bolts like before, placing the card between the pads, reinserting the wheel/disc, pressing the brake lever while tightening the caliper bolts? Then of course remove the card. Duh

It's a brand new bike like only been ridding an hour at most. This rubbing has been there since day one. And yes I've tried that folded card trick as well to no avail. Thanks for the reply though.

First thin is to actually look at the gaps between the pads and the rotor. Lay a piece of bright white paper or similar on the floor and look down through the pads with the white paper helping to highlight the gaps. Spin the wheel and watch what is going on. This will also highlight wobble in the rotor.

Likely the rotor is not centered in the gap.

While ideally you should be able to loosen the caliper mounting bolts apply brakes and tighten the mounts, while holding the brakes. I find this seldom works for me and is a bit bike dependent. Here's the procedure that has worked best for me.

Spread the brake pads which will push the caliper pistons back in. This will create a large gap.
Do the mount loosening thing, but with a brake alignment shim tool. Example shown below. This will do what you've been doing, but leaves a larger gap, after you pull this tool out. It allows the pads to get close, but not all the way there.

https://cimg8.ibsrv.net/gimg/bikefor...6ce4bcfcbe.png
After removing the shim tool, then pump brakes to set the pads to final position.

This works 80% of the time. Sometimes, I simply have to loosen the mounts and manually position the caliper to get the rotor centered. This can be a real pain.

In general, I think the whole problem with this is no matter what, the last bit of tightening the caliper mount makes it move. Probably very dependent on the frame mounts so on and so forth.

Definitely DO NOT drain fluid. The brake caliper pistons are effectively spring loaded with a seal around their perimeter. When the brakes are applied the piston moves and sort of pulls against the seal. When the brakes are released, the seal pulls the piston back. As the pads and rotor wear, there will be enough movement that the seal will slip and not pull the piston back as much. This is a very subtle and gradual thing. And likely what I just said makes little sense.

Bottom line, the amount of fluid doesn't control how much the pistons retract after applying the brakes. But, as I said, pushing the pads and pistons back in will reset the pad position and allow you to start over. Once the pads have moved out (not counting when applying the brakes) , the only way they move back is if you push them back.

If the pistons get dirty, they may in fact, not retract properly. In this case, you need to give them more service. Look online for info on that. But basically you remove the wheel, and pads, pump the brakes to extend the pistons. But not so far that you pop them out. That will be a mess and require adding fluid and bleeding the system. With the pistons extended, you can then clean them. Then push them back in.

Yeah I kind of did this already too. I removed the pads, pushed both pistons in the most retracted position and pumped the brakes until they came out enough for me to clean and oil both pistons and I did this several times in hope of getting them to "un-stick". As it is common, one of my piston is "lazy" and doesn't come out straight away which I've read is ok (Parktool).

I was guessing the issue is one of the piston doesn't want to retract fully is why it rubs and I thought draining a bit of fluid would mean less pressure in the system but I guess that is a wrong assumption according to what you stated.

Thanks anyhow.

Don't move the pistons to center the pads. The normal gap is large enough to see light on both sides.

If you actually observe that the pistons are not moving the same amount on both sides, push them in and restrain the one the moves more easily while pumping the lazy side out with the brake lever. Then push both in and pump out to normal without restraint.

take the bicycle back where you bought it to have them see if it needs to have warranty work.

I would get the dealer to fix it on a new bike.

Is it rubbing constantly? Assume so as you said the rotor is straight. I have usually found that after doing the loosening then braking with a metal shim inserted, I’ve had to deliberately misalign the caliper slightly (anticlockwise when looking at the mounting bolt heads) so that the last turn tightening them up brings it parallel with the rotor. Bit of trial & error and feel. With something white behind on the floor, as mentioned above, you can usually see how far it moves (with the shim out, can’t see anything otherwise) and I’ve also had to stick a thumb in the way to stop it moving too much. One of the joys of disks.
And yeah removing fluid would reduce the pressure. Almost certainly by too much.
New pistons shouldn’t need cleaning. What did you clean them with, out of interest?

It's a Canyon bike so there is no taking it back to fix unfortunately. I cleaned it with mineral oil. I guess I will try again. BTW this also happened to my front discs but I was able to fix it by truing the caliper alignment. Just didn't work for the rears...sigh...

What is the brand/model of the calipers???
Is it a caliper that only one pad moves or both?
The loosen the caliper, squeeze brake lever, tighten, alignment method doesn't always work...same with a card used as a spacer. Often you have to manually adjust the caliper to get it correct and use a flash light/torch to visually see if the clearance is good, rotor is 'true', etc.
Did the brakes...lines, shifters, calipers, etc. come fully assembled or did you have to do any assembly...I'm not familiar with Canyon's assembly process for a direct sale to customer bike.
Have you tried a minor brake bleed to see if there are any air bubbles in the line?

Canyon will pay for you to go to a bike shop. Contact them.

The last time I contacted them it took 1 month for a reply...Their customer service is horrible.

But you got a good deal.

"No good deal goes unpunished"
Or something to that effect...

If the rub is only a small amount that does not slow the wheel down that is not a problem. My hydraulic R7000's had a slight rub on the front it sort hit the pad a small bit but never really slowed the wheel down as such. I did a number of things and really it is not an issue eventually it went away when I happened to replace the pads only because of wear. Note I am a bike mechanic and I did all of the usual things. I tried business cards and cleaned the sides of the pistons, I even did a number of different brake bleeds. Nothing completely removed all the rub. Again it was not something I heard riding the bike only only spinning the wheel checking. Yours might be worse. Let me know I have a few other options.

I managed to fix it after like 3 more tries. I pushed both pistons back in, loosen the calipers bolts and instead of doing the hold the brake and tighten method, I just tighten it slightly alternating between the two bolts visually seeing that the rotor disc are centered between the two pads. I find that if you try to hold the calipers and tighten too much at a time, the caliper will move even though "you think" you're holding it tight. It will still move slightly which in turn will rub again. So in short, just tighten each bolt slightly at a time while holding the caliper tight.

Thanks everyone for chiming in with your insights and help.

That is the correct way. Good job.

I don't know where the "hold the brake lever" method comes from, but it is unlikely to work right.

This is what I run into often. Ideally it should work, but it takes very little force to flex the rotor and it is the rotor that the caliper is clamping to while then tightening the mounting bolts. So, the caliper clamps to the rotor, you tighten the mounts and pull the caliper and rotor one way or the other and then you get the rub. I do think this may have a lot to do with the caliper mounting surface, and likely just how much friction is in the mounting bolts as tightened. I've found some bikes are just worse than others.

Just like you, sometimes I just need to eyeball things and as the mounts start to tighten up, nudge the caliper a bit this way or that.

Atta Boy ! ! !
That's a wrench monkey skill...you only learn by doing...over and over and over until...tada !
Chapeau you are now in the exalted brotherhood of aspiring 'wrench monkey'...welcome to the club

That method also fails to account for the vertical movement of the caliper from where it is clamping the rotor down to where the bolts are snug. And the rotor isn't actually stiff enough to prevent the caliper from rotating as the bolts are tightened. But you can't see it turning because your normal reference would be the rotor gap.

When you get a solution, please post it. Thx

Er, he did?

(apologies paradox, I’m assuming you’re a bloke which is terrible online manners)

I know you have sorted it now, but IME a bit of minor brake rub is pretty common on a new bike until the pads and seals have bedded in a little. It usually resolves itself after a few rides. An hour of riding is not really giving it much chance to settle down. I've had this experience with pretty much all my new bikes over the past decade and only rarely had to re-align the caliper to prevent rubbing. But obviously I don't know how badly yours were rubbing.

Continue to ride it. It will stop eventually.

It's a new bike, it needs time to break in...

He's already fixed it by re-aligning the caliper.

The guy could have probably just patiently waited until his pads broke in properly. That's what I was trying to say.

My bad for not reading up through the posts. Busted!

I don't worry about having a visual gap between the pad and the disc. One piston might be easier for the spring pushing the pads apart to push in and have all the gap on the other side. As long as no pressure is being applied by that pad to the disc it's just as good if it had a visual gap.

No visual gap = rub.

No, you just can't see the gap that's there because it's so tiny.

And even if there is rub, it doesn't mean that it's making noise that one will hear while riding or that it is wasting even a fraction of a watt of power.