If you apply the brakes with the ignition OFF the pedal on my car goes to where I think it should be when applying the brakes.
Turn the ignition to ON and with the engine running the pedal drops too close to the floor with the minimum of effort on it.
Why is that?
The master cylinder piston has travelled very little with the ignition OFF and has to have travelled much farther with the ignition ON. Why is that?

I have a theory but I want other peoples ideas as well.