Fixing hydraulic disk brakes, what to check first?
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.