I recently purchased a used (36K) Mariner Hybrid. Love the car, but about a week after buying it the service brake system lights came on. I took it to the dealer, which proceeded to bleed the system. They gave it back to me and it was fine for a couple of weeks. Then the lights came back on. They took it back in, bled the system again and replaced the master cylinder. Unfortunately, they just called to say that that didn't fix the problem. The dealership has admitted to me they don't know what the problem is and don't know what the next move should be.

Any advice on what to do now? Please help!!!