Some mechanics always look for extra work. I do not know if your master cylinder was bad but you cannot tell by now.
I replace my brake fluid every two-three years. This is a good maintenance habit.
For the warning lights I would check the charging system. Sometimes bad alternator or poor contacts may give you that kinds of behavior. Make sure you do not have any white residue on the battery terminals. Measure alternator voltage output - should be above 13 volts. With the engine running wiggle cables and watch the light - it could be that simple.
Good luck,
Sam