ash d-series

I've read about 10 different threads but none really give me a definite answer.
I've changed the brake lines for braided ones and totally drained the syestem, filled it back up and bled the system with 2 people.
First time i didn't realise the rear calipers had 2 nipples but second time i went round did both rears and the front and the pedal felt great 10 times better than before but as soon as i start the engine the pedal just goes straight to the floor!
I've checked for leaks and its not leaking anywhere 100%
As soon as you knock the engine off the pedal starts to come back its just when the servo is working.
Is the master cylinder faulty or have i missed something?
I've bled it 4 times all together now.
Thanks in advance,
Ash

R0t0rH34d

I've always had to bleed mine with the car running. It seems that since the brake booster uses engine vacuum it won't bleed right with the engine off, or it didn't any of the times I bled mine.