For some days now I am observing the decreasing performance of my disc brakes on my MTB. Additionally, breaking is extremely loud (squeaking). I don't think I have oil or fatty substances on the discs. Still I can't exclude that.

• So what do you think is the problem and how can I solve it?

I would guess I have to change the brakeshoes but I am not sure.

• How can I check the actual problem without wasting time/money?
• If I have to apply new brakeshoes, how is this done safely?