Brake problem!!!!

Right well when engine is off and you press hard on brake pedal the pressure stays firm but when engine is running and you press hard on pedal the pressure starts dropping away and the pedal moves towards the floor. Car has abs with warning light on all the time. Any ideas what would be causing this? I assume master cylinder is ok as you get firm pedal with engine off so was wondering if servo or abs modulator be could causeing these symptoms.
Urgent advice needed please

I assume you have checked for leaks.
My last car (audi) had this, took it specialist who assured me it was normal. Do brakes actually work? A search on't net says this could be master cyl seals.

Could it be boiled/contaminated fluid?

Its the abs if he unplugs it the pedal will work fine as will his brakes seen this on mine of course he wont have abs then though.

New/used rear wheel sensor fitted now abs works as it should and had system properly bled at garage and brakes are spot on now.