This guide has been around for a few years and has rescued quite a number of brakes on SDH and on UKMB. There's nothing revolutionary here, just good practice.
So, your brakes aren't stopping you? It helps if you understand your brakes a bit first.
Your brake pads don't brake against the metal of the brake rotor, they bite against pad material that's embedded into the brake rotor surface during the "Bedding in" period ( Debatable if this actually happens on MTB disc brakes). So, if you contaminate your brakes, you've got to remove that pad material. In most cases, hydraulic problems are felt/observed at the lever whilst Braking surface problems affect your stopping distance.
1. Is your bleed OK?
If your lever always pulls to the same point and is firm, your bleed is fine. If it is squishy or constantly changes the bite point, you need to bleed. (always remove your brake pads and wheels whilst bleeding your brakes!)
2. Still not working? Time to clean your brakes.
Are your pads Sintered or resin/organic? For resin/organic pads, use the no heat method, for Sintered pads, use the heat method (bottom of this guide).
No heat method
3. Clean your rotors
a) Remove the pads, put them to one side, preferably somewhere clean, pad surface up!
b) Obtain rotor cleaner (either proprietary or Isopropyl Alcohol).
c) Spray on your rotor cleaner, then use clean, lint free rag to wipe the braking surface. Do this a few times with a fresh bit of rag each time until it comes up clean. Just spraying your rotors with cleaner does less than nothing.
d) Re-spray the rotors. They should be clean now. Try to keep them that way.
4. Clean your brake pads
Look at your pads. Chances are they look black and glazed. You don't think you've badly contaminated them so let's try to clean them up.
a) Degrease your pads. Two options:
......i)Spray with brake cleaner and wipe.
......ii)Pro tip from the very nice mechanics at Glyncorrwyg: Add a blob of Swarfega Original to one pad, apply the other pad and scrub them together for a few minutes. Then rinse. The results are fairly spectacular!
b) Give the pads a light sandpapering to remove the top surface.
c) Re-fit your pads and bed your brakes in. Don't chuck dirty water on your brakes. Don't get them hot then cool them down. Just do some nice hard stops from quite fast. That should do the job.
If, after all that you have firm, clean, bedded in brakes that'll stop a train. If you don't, your pads may be more contaminated than is rescuable in which case, clean the rotors again and replace the pads with new ones.
Heat and detergent method
1.Degrease the rotors.
Don't heat them up or you'll ruin them.
a) Degreasing involves more than just spraying on brake cleaner or isopropyl alcohol. You need to wipe them thoroughly with a tissue or lint free rag soaked with cleaner. Use bog-roll or kitchen roll as facial tissues contain oils.
b) Make sure that you use a clean bit off tissue as soon as it gathers any muck.
c) Wipe until your tissue comes away clean.
d) Make sure you've sprayed and cleaned both sides of the rotor (easily forgotten)
d) When you're done, give the rotor a quick spray on both sides
2. To restore the brake pads (assuming they're not too badly contaminated),
a) Degrease them with a detergent. I use fairy. Apply a blog of fairy liquid to one pad, and use the other to grind against it, pad surface to pad surface. When they feel fairly degreased, rinse thoroughly.
b) Then, using pliers, hold them in a blue gas flame (hob or blowtorch) until they stop burning yellow. I'd keep em in the heat for another minute or so after that. Don't use the most fierce setting on your blowtorch, A gentle blue flame is all you need. Do not use a yellow flame, you'll soot up your pads.
c) Let them cool, then refit. Avoid the temptation to quench them, you might damage the backing material.
They'll need a little re-bedding in, but they should work pretty well. It's cheaper than buying new pads at £15 a shot, and if your rescue attempt doesn't work, you've lost nothing. This is a last resort though. Buying new pads is always the safer option, but be sure to follow my rotor cleaning guide if you do change your pads.
I've had consistently good results with this method over many different brakes.
Different methods work better on different sorts of pads. The swarfega method worked much better on organic/resin pads than my saint sintered/metal pads, I would not recommend heating non-sintered pads, but if you've got a badly contaminated set you fancy trying to rescue, please give it a go and let me know the results.
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