1976 240d brake problem

[240Dude]

for the past few days if has seemed like my brakes were fading. traffic has been bad and though the pedal was a bit longer it wasn't much. i thought i would just keep an eye on it. well today the pedal goes almost to the floor. no leaking fluid... not low on fluid... super spongy feel, so i figure bad master cylinder but if i pump the brakes i get a super hard pedal like there is no booster. i am going to check the brake lines and pads to see what shape they are in and even though i hate bleeding brakes i'll do that too. how can i check the booster? are the mc's for sale on fastlane new or rebuilt?