i have replaced the master cylinder as it was leaking down the booster. bench bled the master befor installing. bled from all four wheels. i took it for a run around the block and all was good. left for 5 minutes and could push brake pedal to floor(required some pressure). rebled all wheels had air in LHR. pedal rock hard. when i start the motor pedal feels soft and spongy. bled all wheels again no air. ive checked all flexible hoses for leaks or swelling all ok.
Any ideas

I remember reading somewhere that on a standard B-series front caliper, depending how the car is setting, etc, etc, the bleed nipple may not necessarily be the highest point of the caliper when bleeding. So air may still be trapped in the system.

In some cases you have to remove the top caliper nut and swing the caliper backwards so its vertical, put something between the pads, then do a bleed. This should get the nipple at the highest point on the wheel.

Not sure how true or accurate this is, its just something I remember reading a while back about someone who had a hard time getting rid of all the air from his brakes.

Also assuming you've bled the things correctly, as in longest line first, etc, etc.