Bad master. As in - piston seals or O-ring, whatever it has, blows fluid by.
That's fine, but with power brakes, the helper needs to pump the brakes with the engine idling.There is two ways I always done with brake work.,
First methold which you will have a helper to help you on this one what you will do is pump the brake until firm and have a helper turn open the bleeder valve but a little a time and repeat as need to be and MAKE sure you do NOT let the fluid go below half level otherwise you will restart the whole thing over.,
Start at the furtherst one first useally right rear most case then left rear then right front then left front which this is most common and also a tip if everthing is working good but feel uneven blance on brakes what I do is crack open both front and back and hit the pedal not much but just enough to get the equizer valve back in place.
That methold is used with two peoples but if you are working alone you could buy a one way bleeder valve which it will include a check valve inside of it so you can pump couple time but be aware how much fluid it will go down so pump no more than two time and close the bleeder and refill the brake cylinder tank.
The other methold I used from time to time if you are not super hurry what I do is fill the brake cylinder and let the gravity take care of it for a while but keep your eyes on the master cylinder to not let it go below half level I did mange it few time before ( this is one of few metholds you really have to know what ya doing )
Hope that help ya with the info.
I bled a few brakes over the years, and only a year ago, replaced a caliper on my vehicle. Never have I have the engine idling while bleeding the brakes, and the GM Service manual makes no mention of it. My peddle feels like the day I drove it off the lot new. My experience.That's fine, but with power brakes, the helper needs to pump the brakes with the engine idling.
The pressure head adpator that can fit top of the master cylinder will do the good trick with it ( useally work very well with non ABS but with ABS system it will be little differnt ) and with ABS you may need to get a adpator or speical tool to cycle the ABS pump / pressure resvoir (tank )There's no need to have the car running, to bleed the brakes, if the car has power brakes. Once the vacuum is released from the booster, you've got direct contact with the MC piston, bleed away....that being said, I'm strictly referring to vacuum assisted power brakes, I have no idea if a hydroboost set up requires a different procedure.
The Hydroboost verison do use the same procedure as convental vaccum booster is. So there is no differnce on that due all my trucks I have are diesel and used Hydroboost system.
The mention of ABS is spot on, as that can impact how brakes need to be bled if the lines are cracked pre-ABS module. One of my cars has a specific tool for that, it does that among other things, and it's pretty expensive for a typical DIYer to run out and buy. The fact your pedal keeps going to the floor means one of two things - a massive amount of air in the system getting compressed, or as mentioned prior, a leaking MC piston.
Another option, if any local parts stores have them for rent, is to get a pressure bleeder, and bleed again to see what comes out. When used properly, those make the tedious process of bleeding brakes a walk in the park.