Ignore the suggestions to use cleaning fluids etc, that won't cut it. If you have already ridden the bike without bedding the brakes in properly, you'll have to start again with brand new disc pads, no point trying with ones you've already used as there's a thin layer of material on fresh pads which has to be transferred cleanly and evenly to new or freshly sanded down discs (if sanding down, make sure you DO clean them with white spirit or isopropanol and a clean lint free cloth). If you don't do this by the book, you'll never get optimal transfer to the discs. Good news is you don't need to bin the existing pads, just save them until the next pair are worn. You can ask a good bike shop to do the bedding in process for you, but personally wouldn't trust anyone but myself to do this right as some shops don't know or care about it. You need to get the bike up to speed without using the brakes, then brake hard but without coming to a complete stop, accelerate again and repeat 10 times. Do this process for each wheel. This is easiest to do in an empty car park or on a long descent if you have one nearby, or do it on the open road with care of motorists around you. Before I appreciated all this, I unwittingly botched the bed in process on the rear wheel, but after some hard braking on a downhill stint, managed to get it near good enough, but it was never right until I started with fresh pads. Few people realise how critical the proper process is to getting the best out of their brakes from day 1.
