soft brake pedal

Well got my mustang going again last week after sitting for about a year because I have to sell it. Went through the cooling system, etc to get it ready to drive again. Bled the brakes so that they all have brand new fluid. Took it for a drive and the pedal feels pretty soft with not much braking power. The master cyl. was replaced not long before I bought the car. Probably is 2.5 yrs old with maybe a 1000 miles on it. Wondering what the problem could be? The pedal is very firm and pumps up when the engine is off. But as soon as I start it, it feels softer(normal) but even when coming to a stop in 2nd gear I am on the floor.

cliffs: soft pedal only with engine running
bled brakes
problem got better but not resolved
master cyl only has maybe a 1000 miles (2.5 yrs old)
stock front brakes with fresh pads and rotors
disk brake rear with fresh pads and rotors
No rubber brake lines, all braided or hardlines

Thanks for the help in advance

ok, with the engine off, I just now depressed the pedal to see how long it would take to become "hard". I could not get a full stroke before the pedal became hard, released the pedal it is hard immediately. This is normal. I'm just not sure what to try next