rear brake help needed

[1999audia6]

i just did the rear brakes on my car its a 99 a6 quattro. they had clearly never been done. When i got the car they would grind terribly then i was driving and all of a sudden i had no rear brakes, i would have to push the pedal all the way to the floor and my front brakes would work for me but nothing in the rear. when i pulled them apart i found that one of the pads on the passenger side had fallen out, it was nowhere to be found. I replaced both rear rotors and pads and bled the brakes the whole 9 and it still does the exact same thing. when i pull the e brake they lock up so its not the calipers. its not the lines theres no leaks anyewhere. When the cars off the pedal is solid and feels like it should but as soon as i turn the car on i lose it. Im totally stumped i desperately need some help, let me know anything i should try. thanks

[MP4.2+6.0]

Probably master cylinder if it wont hold firm pressure (it sinks over some seconds). Turning on the engine may just be giving you the power boost you need to put a lot more pressure on the pedals. If it isn't leaking anywhere but pedal sinks pretty much has to be the master cylinder.

One other thing to try, but likely not it. Drive car in reverse and apply brakes several times. That would help tighten up the rear automatic brake adjustment and may yield a bit firmer pedal. It will adjust over time anyway as you drive, but that can speed it up.