Brake pedal goes to the floor

[RS_X5]

I've just changed my front discs and pads. Now I'm unsure if this was happening before I changed the pads but when the engine is running I can press the pedal all the way to the floor, its hard to do this but it eventually goes down.
With the ignition off the pedal is rock solid near the top, but as soon as I start the car I can press it down to the floor.

The brakes work still even when they are on the floor.

Is the a problem? And what could it be.

I don't think there is air in the system as with the ignition off It would be soft.

Please let me know