Brake Pedal Goes To The Floor When Sitting Still.

[Flat6]

My wife's '94 ES300 has 103k miles and the brakes give me some questionable action when sitting still. If the car is running and I press the brake pedal, it will go to the floor if I keep pressure on it. However, if I shut off the engine and press the brake....it's solid. If moving and hit the brakes, it stops on a dime. Just wondering if that's normal action or if the m/c needs rebuild/replace. Acts like there is a loss of pressure by the brake fluid leaking around the seals/cups in the m/c when you just sit and press, but a sudden pressing on the brake doesn't allow the leakage. Any suggestions, thoughts, etc., on this?