[sleepertruck72]

A quick way to rule out the master cylinder is to undo your brake lines, and plug the m/c lines, your pedal should be rock hard. But if you are having to add fluid because the brake light is coming on, then possibly your wheel cylinder(s) is leaking, causing the pressure differential switch or low fluid sensor to activate the brake light. If you have air in your lines, that would cause the low brake pedal also.