My disc-brakes on my mountain bike sing/scream when I brake. I have tried to clean them, but it didn't work. Should I replace the disks or the brake pads? Is there a better (cheaper) way to fix this.
In short, squeaking brakes are caused by the sound of your pads sticking then slipping increadibly quickly, rather than applying presssure to your discs smoothly.
Solution: Sometimes these problems can be a right pain in the butt. Sometimes your brakes need time to bed in, sometimes, as you know, your discs just need cleaning, or importantly, drying. But if that doesn't work you need to rough up your discs. You can get products with abrasives in them just for this purpose, or, less expensively.. I know many people have had success with very fine sand paper.
Give that a try before replacing your discs.
Here is a tip for you - putting a smudge of copper grease on the BACK of your pads can reduce the noise. It may still be there until the brakes are properly bedded in, but I have found it does make a difference, especially under lighter braking.