Last week, it seemed like the master cylinder was going out not more than a year since it was replaced. I replaced it again under warranty but have had no luck at all getting the brakes to work again. I thought that I might have gotten a bad new unit so I was able to swap it with a new one yet again.

I'm on my third MC in just one week and I'm pretty sure by now that this isn't my problem

Car off, I can press the brake pedal almost all the way to the floor. Car running, I can floor it with some effort. It gives all the indications of air in the system but I swear that I know how to bleed the brake system properly and have gone through two large bottles of brake fluid. I tried several methods of purging the air including vacuum through the wheel end.

The only thing I'm not sure about is if a bad brake booster could cause this. I tried YouTube-ing it but can't quite get a satisfying explanation of what's not right.
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