It's not uncommon for a brake pad to become contaminated with grease, oil, or just plain dirt. This can build up and start discoloring the rotor. Just take off the caliper and clean the rotor with a clean cotton rag and some rubbing alcohol. Then pull the pads out and sand them down with some 100 grit sandpaper. I like to chamfer/bevel the leading and trailing edges which helps reduce noise.

 

also check to make sure a small particle of brake pad didn't get 'welded' to your rotor, which can be removed with the sharp point of a box cutter followed by a light sand and a clean with alcohol...

 

Look closely at the rotor. Is it a simple discoloration or is it a burn mark?

Either way, lightly sand the rotor(s) and pads to supply a new surface. That "mud" you smeared on the rotors may have lodged a partical or grain into the pad(s) which may have scorched or grooved a section of the rotor(s).

If it appears to be a burned mark, check the pad clearance. I roasted a rear rotor recently due to a stciky piston which kept a very slight but constant drag on the rotor. Overheating occured and it got noisy.

Also check to see that the caliper is centered over the rotor.

Hydraulic brakes...fun stuff :lol:

 

Yeah, loads of fun . . . so far these Hope's have been more trouble than the Avid's, but only because I'm kinda new with Hyrdo's and am trying to do this all on my own. I'm sticking with them though because when they work, man do they work.

Hawg, this was the front . . . no mud there I did learn though that its off center by about .4mm (using a feeler gauge. Am I neurotic? Yes.)

With the rain today (and still no back tire) I'm going for Dim Sum and then gonna mess with these things.

 

I made two discoveries this evening . . . one is that 1 of 4 pistons on my M4 was sticking (slow to extend), and it was on the side opposite the streak which makes me think that the side opposite the slow piston was working harder, hence the black streak.

I also discovered that something must have gotten on my pads. I lightly sanded the surface of the pads and they left some real dark stuff on the sand paper too. Everything's back together now, new tire too and I'm ready to ride! Oh, wait . . . its sunday night. Damn.

 

Thanks for the update. I suppose that is part of the price to pay for more pistons - the chance that some of them don't work the same. Good find though.

Hopefully this has the brakes sorted for you now!

 

I hope so too. I really want these brakes to work to their potential . . . I think I'm on the right track at this point. Hope sent me some new diaphragms so I replaced those too . . . so basically, I'm back to square 1 with these brakes, but with a LOT more knowledge about how to properly set them up and fine tune them.

Shalom, since I know you also have the same brakes, do you have any tips on centralizing the pads? I've been using a small screwdrive to leverage the backplate of the pad opposite the one I'm trying to move and noticed that I'm marking up the caliper body. Not bad, but I don't want to make it worse.

 

And when you say "adjustments" what exactly are you referring to?

What I have been doing is squeezing the lever, watching the pistons move and spinning the wheel. If the rotor flexes at all when squeezing the lever, I push those pistons back in as described above to "reset" them and then squeeze the lever again to bring them back closer to the rotor, but allows the one that was further away to get there first, if that makes sense. Hopefully I've been doing it right . . . seems to have been working.

 

After aligning the caliper, I have been squeezing the lever until the pads contact. If I remember correctly, the pad that contacts second is pushed in towards the rotor a little bit (this will allow the pad that contacted first to in essence be pushed in).

Keep doing that until they are centralized.

I gave up looking for rotor flex and just made sure that I could see which pad contacted the caliper first using a large sheet of white paper under the wheel and squeezing the lever really slowly.

 

I think I've been doing it wrong! Well, I'm going to try this evening . . . I'll report back!

 

Best of luck

 

Got it sorted out . . . now I just gotta true the damn thing. I actually DON'T think I was doing it wrong, but I do think that I wasn't taking enough time with it. I also think that a) the sticky piston and b) the caliper being slightly off center didn't help.

I'm thoroughly expecting good things to come

 

Glad to hear that you are getting there. Once set up right, they are absolutely amazing - as you are well aware.

 

007, the 'dark stuff' is probably brake dust. personally I prefer to soak my pads in denatured alcohol and clean with a stiff bristle brush - will air dry in minutes, sanding can glaze the pad... I'll file and sand my rotors though - again followed by a thorough cleaning in denatured alcohol.

 

How would I know if I have glazed the pad? I didn't press hard and used 100 grit paper.

 

Glazed brake pads will look something like these

Note the outsides look normal, but the middles are glazed - gets smooth, shiny and hard.

 

if you need to mill down into the surface of a brake pad to remove contamination (usually from something oily and viscous - not usually dirt, grime or brake dust) I've found it more effective to just replace the pads . It is possible to sand into the brake pad surface just make sure you scrub and clean vigorously, removing the contaminated particulate or else the heat and friction generated in the brake system will just glaze it--you'll know if that has happened, super reduced braking/noise... I've found there are few absolutes, mountain biking is a very dynamic sport, and with experience, over time you'll amass a broad knowledge base... hope this helps