Fixing hydraulic disk brakes, what to check first? I have a 10-year old bike, fitted with hydraulic disk brakes. The bike has been actively used for around 4 years only though.

Recently, my brake levers started to have more and more "play" before the bike actually starts braking. A kind of a "dead zone" where nothing happens, just like if there was not enough oil.

I went to the store to fix it, twice, but it keeps coming back. They say something may be stuck ("bourrage" in french, as ugly as it sounds). They told me they bled the brakes, but at this point I'm not sure anymore.

So now I'd like to fix this myself, once and for all.

The brakes are the only thing I've been afraid to fiddle with, so far, so that's why I'm coming here for advices.

Is there any "fixing your hydraulic disk brakes 101" or equivalent? A guide I could follow to check things in order. Shall I check brake pads first, then oil, then... A flowchart would be lovely!

tl;dr: What to check first to fix hydraulic disk brakes?
If they are SRAM/Avid hydraulic brakes, you're screwed. The DOT 5.1 fluid attacks the o-ring seals, and makes constant issues. Storing the bike for a while, upside down or vertically, will also cause issues. If they are Hayes or Tektro, same deal with the the DOT 5.1. This means don't spend money on them, replace with a Shimano M596 or better. They use mineral oil, which is much better with rubber, moisture, and expansion. They also have the fewest issues of any other mainstream product out there, by far.

Now, SRAM currently allows a no-questions asked full replacement warranty with all current models. I don't recall how far back it applies. If 10 years, maybe not. But, still worth a try. Ask a reputable LBS to make the call to SRAM on brake warranty. Thing is, you're getting another SRAM brake, which will do the same thing again.