i had bleed the entire brake system with the car not running, the pedal presssure and height was perfect!! but , when i started the car my pedal goes to the floor??
if i rev the engine constant at 2000-2500 rpm the pedal is stiff again until the rpm drop to idle , then i have no pedal??? :?

Sounds like you joined the "My master cylinder is dead" club. No amount of bleeding will cure that issue...repair by replacement

brand new mc ??

I dunno....is it??

If it is, did you bench bleed it before installing the lines to it? It is also possible to get one bad from the box...esp. if it is a reman.

off course it was bench bled ,

could it be the booster?? this is why i like old cars,,pre 1980