If the brake pedal goes to the floor on light pressure or reasonable "normal" braking applied pressure, at standstill engine switched off ,then apart from air in system or fluid leak I suggest it probably is the Master Cylinder especially if the fluid stays at the same level in the fluid tank after releasing the brake pedal,
Now forgetting the brake pedal going to the floor if it's spongy with Vacuum in the servo ie. you have had the engine running, to test if there is air in the system .Switch off engine pump brake pedal , the pedal should become hard ,if it doesn't then possibly it has air in the system ,or as above Master Cylinder , or as in one case I had in my garage after wrong diagnosis we replaced the master Cylinder , because the Brake pedal crept to the floor , we remained the vehicle ,and found very slight play in the non adjustable front wheel drive bearings on the O/S it turned out to be the" outer" part of the bearing was turning in it's hub , we replaced hub and bearings ,problem solved , never met this problem before .
My advice is don't guess get professional advice not form your mates or down the Pub etc. take it to a Fiat Commercial Dealer it will cost , but if brakes fail you could be a long time dead!!!!
ps. I swallowed the cost of the Master Cylinder but the customer kept the cylinder on his car, from memory that was approx. £130 (trade price)
The above is what can happen or how it could be , but, if you suspect you have a problem take to the Professionals please
Tony A.