Grinding rear disc brake
 

Hi - After a very hard mtb session, my brand new, shop-installed SRAM Level hydraulic disc brakes started grinding in the rear. This is taking place ONLY under braking, it's quiet when not braking.

The sound is like a vibrating grinding noise, kind of like the pads are grabbing each individual hole in the rotor.

Is it possible the rotor/pads became contaminated?

The only two things I can think of is:
1) I had the thru-axles out this weekend and I lightly greased the threads before reinstalling to prevent seizure. It's not probably but I guess possible some stray grease contacted the rotor
2) My ride this AM was very technical and I took some rocky lines quicker than normal, I actually bent my front rim a little when I hit a small rock I didn't see, in the trail between two TTFs. When I stopped, I noticed my tires lost a little sealant, I suppose it's possible some sealant fount the rotor?

Thanks guys!

Car guys often talk about "bedding" brake pads to prevent "rotor warp". Braking heats up the pads causing them to leave a layer of pad material on the rotor. If you brake hard than come to a complete stop this can leave a spot of extra thick material in one spot. The brake action is no longer smooth, causing the brakes to pulse. On a car this causes the steering wheel to shimmy, like the rotors are warped.

Try cleaning the rotors with solvent and taking some fine sandpaper to the pads. If that doesn't work remove the rotors and hit them lightly with a sander as well.

That's a really good point, I actually have experienced this when I worked on a pit crew and the driver panic stopped and locked up the wheels, sometimes you'd jack the car up, spin the wheel and it would stop before completing a full revolution!

Just a quick follow up - I think that's an excellent place to start - do I have to get too scientific with the sandpaper? I have some cheap stuff that will work if not, if so then not a problem. I can definitely hit the rotor with some alcohol as well.

One guy on my FB group suggested burning the pads with a propane torch to burn off any contaminants? That sounds like a really good idea or a really bad idea.

Thanks so much by the way, I am new to disc brakes and this is only my 3rd ride with these new hydro calipers!

Also, since they were just installed you're probably wondering why I don't contact the LBS who did the work for their take - They're closed Mondays and they're also super backed up so if it's something like you mentioned like taking apart, cleaning/sanding, reassembling, then I can do that no problem! I actually only had them install because there was an issue getting me the calipers I wanted and they felt so bad for the delays they insisted on doing it for me, plus they have all the bleeding do-dads that I haven't yet collected.

I've always used a garnet paper, I think 180-220 grit.