Contaminated rear brakes (Shimano)


2018 Fantasy Football Toilet Bowl Lead Technician
Team MTBNJ Halter's
Has anyone had an issue where their rear Shimano brakes get contaminated and lose almost all stopping power?

I have some cheapo deore brakes that when working properly are really all I need on this particular bike.

The problem is that when the bike sits for a while, something seems to be contaminating the rear brake pads. It's almost no stopping power, and if I try to pull harder they just skreach loud and still don't help me stop.

The last time I was super careful and made sure no chain lube got on. It must be the mineral oil but I don't see any leaks.

So, has anyone else had this? And if so, is there a way to de-contaminate the pads?


Formerly 1sh0t1b33r
Team MTBNJ Halter's
Brake clean on rotors, no problem. Pads are tough. The only thing that really worked for me is burning it off with a torch to boil and evaporate it off. You may release the glue holding it to the backing though if you just superheat them. I've done it successfully a few times and got rid of the squeal. If it's a your B-bike, just get some cheap pads.

As much as I try to keep them clean while bleeding, there's always something. I need to start removing the pads every time. Otherwise, no actual leaks that I can see.


Master of the Metaphor
I’ve had two issues sort of like that some time ago... first time I did a piss poor job on my bleed and left the bleed valve open a smidge. Ruined my brand new pads. :mad:

Second time I started to get some weeping around the pistons. Since it’s over time maybe it’s the latter?


Active Member
Baking them in an oven worked for me. I think I did something like 350 for 20-30 minutes and then let them sit in the cooling overnight. Might want to do the same with the rotors as both are probably contaminated.

Before that the were squealing and not stopping well. Afterwards, like new. Almost forgot, also did a light sanding of the pads afterwards.


Well-Known Member
Team MTBNJ Halter's
I've had it happen to my XT's before. Try everything. I used both Brakeleen and blowtorch...but nowhere near each other obviously. Do both the rotors and pads. You can also sand down the pads. Sanding the pads after cleaning and torching them actually worked pretty well for me.
I recently replaced a 5 year-old set of XT brakes for this same reason. I went through a few sets of rotors and pads in the last few years trying to solve this issue, cleaning/sanding and/or flat out replacing. That would temporarily remedy the situation, but like you if I let the bike sit for a few weeks there would suddenly be awful howling and significant power reduction.

Interestingly, when riding lift-access the brakes might start off feeling like crap, but half-way down the first run they would feel great again. With higher speeds and brake temps I figured whatever was getting on the pads/rotors was able to burn off, the rest of the day they would feel great. Sure enough though, if I let the bike sit for a while after that the same issue would come up.

I figured the brakes were just old and seals within the caliper itself were leaky and some tiny amount of fluid was getting on the pads/rotors. Hopefully my new 4-piston XT's don't have the same issue.
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Well-Known Member
I am having a similar issue currently but just the rear brake and who needs that anyway so haven't looked too much into it, plus no time with work lately. when I have the time gonna drop it off at the LBS and have them take a look.