Ba Xt Brakes help please

I Have a 2004 ba xt auto 6cyl.
when the engine is not running, the brake pedal is firm, when the engine is running the brake pedal is spongy and goes almost to the floor. this is what i have done so far:
new front disc's and pads, checked rear disc's and pads all ok there, bleed the entire brake system to all wheels.checked the vaccum line to the booster, all seems ok there, there are no brake fluid leaks anywhere.
the brakes work fine when on the move, but when you are stationary with foot on the brake, the pedal goes almost to the floor, and when you are on the move again all ok
any help will be appreciated.

Sounds like the Booster Check Valve is shot.
Go to your friendly Ford Parts store and get
a new one, part number is 8R2Z 2365 AA $26.50