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F-150 Brake problem

Hi everybody. My sister in law has an '01 F-150 4x4 with disc brakes all around that had a fading brake pedal. The brakes seem to be working ok but the pedal slowly goes to the floor with the motor running or not. I naturally replaced the master cylinder, but it did the same thing. Thinking I got a bad unit I took it back and got another one and installed it. You guessed it, same thing. I cant find any leaks anywhere, no indicator lights, no vibration. Im confused. Ive done many brake jobs and this is the first time Ive run into this. Any thoghts? Thanks.

Does this truck have ABS? If so you might have a problem with the hydraulic module. Verifying that the leak is in the unit will be difficult. Throwing a part at it will be expensive.

Let us know how this comes out.

Remove the brake lines from the master cylinder and plug the outlets. Pump the pedal several times and then hold constant pressure on it for a minute. I once found 12 new voltage regulators to be defective.

yeah its got ABS. No ABS light on the dash. I will try the plugging the lines if I can find something to plug them effectively. I hate to give it back to her without it being fixed. Thanks for the ideas.