Brake pedal goes to the floor randomly, HELP

What is your question?

My sisters brake pedal goes to the floor every now and then. From what I read it most likely seems that it has to be the master cylinder, please correct me if I'm wrong. There's no brake fluid leak and no air in the lines. If the Brake booster was bad it would make it hard to press the brakes, correct? Also, on this year car can you tell me what the brake bleeding order is(Ex. RR,LR,FL,FR)? Thanks in advance.