How does a broken return spring allow your pedal to go to the floor????????? You are saying that if I didn't have a return spring, that my brake pedal would hit the floor? No way!!!!
Two different systems.....one hydraulic, one mechanical. Second one assists the booster in returning the pedal...nothing more.
Youse got bigger problems me thinks!!!!! (Bad master cylinder!)
Fix your brakes, then worry about the spring. The world will be a safer place!

Steve
PS: 52K on my original spring....