I bought myself a "previously enjoyed" mountain bike that has disc brakes.
This is pretty standard now on mountain bikes. The stopping power is pretty awesome, and they aren't sensitive to rain and muck like standard rim brakes. However... with new technology comes new challenges! I inadvertently squeezed the fluid out of one of my brake calipers and had to replace a piston on the other (I broke it).
To get the brake pressure back, you have to do what's called "bleeding". This is simply pumping in new fluid and getting the air out of the system.
What You Need
My brakes are Hayes HFX-9's, so I bought the Hayes Bleed Kit. It's about $20 online.
This great video and the Hayes Bleed Kit Manual got me started, and they're an absolute must before you start!
Rather than go through the whole procedure, let me talk about where I deviated from their instructions.
- When you start squeezing in the fluid, you have to let the bottle suck out the air as it returns back to form. Do this a bunch of times until all the air is out. It also helps to tap the calipers with a wrench to release any air stuck in the caliper.
- When the bubbles look like they're gone and you think you're finished, apply pressure on your bottle, and sloooowly squeeze the brake. Release, squeeze more fluid in, then repeat. I did it 3 times, until I felt a bit of resistance and all the bubbles were out. This was critical (and NOT in the video!)
- My first couple of tries, the brakes were squishy and didn't work. Tapping the calipers seemed to help get more air bubbles out, squeezing the brake lever helped... one other trick I saw (and tried) was to re-plug the master cylinder plug before tightening the bleed valve, and squeezing just a bit more fluid in. Try, try again...
- My bleed valve on one brake was leaking like an S.O.B. when I would squeeze in fluid, and it seemed like air was getting in through it. I removed it entirely, taped it with silicone thread tape, and screwed it back in. That solved the issue, and I had much better success after that.
- DOT 3 and 4 fluid are interchangeable, and you DON'T need the overpriced Hayes stuff... go to any autoparts store and you can get a liter of DOT 3 for like $10. It'll last you forever.
Have fun... and if you get yourself in over your head, just go to a bike store! $20 a brake and you're good to go.