My brakes are real unresponsive and it really takes some force to brake effectiveley. The pedal pretty much goes all straight down with very little resistance and a little squelching noise from the arm of the pedal itself. There is the proper amount of brake fluid, front brake pads have about 70% and rear pads have about 50%. This concerns me as it might start snowing next week. Also my car wont shutoff when the engine is still cold. I am assuming this is all vaccuum related. So, I have some questions for those inclined to answer:

1.With a vaccuum gauge, where and how do I test the functionality of the brake booster, vaccuum pump and shutoff valve?

2.How would I know if this is the master cylinder?

3.What else could this be attributed to?

4.What is a typical brake boosters life expectancy?

I really appreciate all the help everyone has provided on this board.

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1983 300SD White with Grey interior