Brake Help

The rear brake on my Dads bike has developed a nasty howl. Shimano SLX lever, XT caliper, 2 piston. 180mm centerlock shimano rotors.

I've done everything I can think of, so I need some new ideas. Done so far:

- sanded the rotor

- sanded, cleaned (rotor cleaner) and re-bedded the pads (1 instance of each)

- Replaced the pads and then re-sanded/re-bedded after the howl continued

I'm wondering if the rotor is finished because I can't think of what else to do. Burning and turfing pads is getting expensive.

Sticky piston?

A howl like old Avids when they get wet? Is the alignment of the caliper with the rotor ok?

Finned Ice Tech pads? 

The rotor has a minimum thickness stamped on it. Measure that with callipers. 

Pull the pads, fill the bleed funnel with fluid and install, dip a cotton swab in mineral oil and wipe it around the pistons and piston seals. Move the pistons in and out. Your cleaning and lubing the pistons and seals. Wipe excessive fluid with a clean cloth. Rubbing alcohol will cause the seals to swell  

For Shimano brakes I do a combo of the Marsh bleed and the Park Tool service video.

I’ve also started burning my pads on the BBQ. Seems to work.

Go faster, sound should stop

Full bleeds done, done the piston lube - and yes alignment is perfect.

No, I have not done the rotor thickness check

brakes just slow you down

aside from the howl, is the power still there?

I had a similar problem with xt's, although they were down on power too. I did all the things you did in the original post with no success. I thought I had bled them well, but did it again and I swear I flicked the level a 100 times in all different positions at the end of the bleed and wouldn't you know, a wee little bubble came out and they were all good again.

I had a not very old XT caliper howl and lose power no matter what I did.  Sort everything out, felt good on the stand and around the block, terrible by mid ride, unrideable pretty quick.  Velofix traced it to a slightly cracked piston, which would contaminate everything after a few hard applications.

thanks all - the power is still there, just super loud.

Next time I have the bike in my hands, I will look at the pistons carefully - that's an interesting thought.

Geez, it sure does sound like contamination? Only time I've had that was when the pads were exposed to contaminant, sometimes the power was still ok. But then again the pads were replaced...Is he spraying aerosol onto the bike anywhere maybe?

Posted by: Hepcat

Is he spraying aerosol onto the bike anywhere maybe?

He'd better not be!

I had an issue recently where my rotors would vibrate and howl at high speeds when hard on the brakes. Definitely not a contamination issue. I ended up replacing the cheap generic rotors with Shimano rotors (not ice tech) and everything is good now.

I've found getting the brakes wet, muddy and gritty seems to help with howl, it's easy to try.