Mk5 GTI Brake Issue

Hi guys, any help on this would be greatly appreciated. My sisters Mk5 GTI Golf has developed a brake issue over the last week. No work has been done to the car recently. The brake pedal is hitting the floor only when the car is going. No leaks, good pads and rotors. With the car off brake pedal can be pumped and good brake pressure is maintained but as soon as the car is started it all goes to shit. Free play is about 3/4 pedal travel then bites and slowly sinks to the floor. Ive never seen this on any other car. Thinking an ABS issue, any ideas?