How can you tell if master cylinder is worn out?

Start the car, place you foot on the brake and push and hold it. If the brake pedal very slowly sinks toward the floor, the master cylinder is bad. Of course a leak at the master cylinder means it is bad. Don't ignore any of these 2 symptoms as it can stop working at any moment, and leave you with no brakes. When looking at the master cylinder look at the wall or booster that the master is bolted to. If u see sweating, or paint pielling at the mating the master is leaking from the seals.