Pops 300e: Brake Problem...

I'm driving my 82 y/o father's '86 300e for a couple weeks. 170k mi. The car runs / rides great except it has one serious safety issue.
After starting and pulling away (ie pulling out of drive way) , the car
has almost no brakes. I'm 6', 220 lb and pressing hard on the peddle has almost no effect on the braking. I've found that pumping will kick in some braking but I'm concerned for my pop's safety. The brakes work normally after the car hits the road for a few minutes.
Thoughts appreciated.
Chip L
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Chip L wrote:

It needs a new brake booster or the lines need bled
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Possible, if there's a hissing noise it's the booster. But these really don't fail very ofen even going back as far as the 50s. M/C's fail decades defore the booster does from what I've seen and in the general case (which of course by no means ensures a correct diagnosis here! I'm just saying what's most likely given the symptoms).
It could be the lines and if the brakes were just worked on it'd be my first guess but they really don't just open up like that and leak.
Also usually air in the lines and a bad booster give pretty constant symptoms, of they're all over the place weird sumptoms agaim I suspect the M/C.
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Brake booster.
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Sounds very much like a master cylinder failure to me. Not a terribly expensive part and you got nearly 25 years out of the old one.; now it's time to chnage it.
Given the age of the car, if you plan on keeping it change the booster too. While some may last a while (a long while) there's a different feeling running around in a 25 year old car with all new brake hydraulics as there is running aruond in one with the original quarter century old parts.
Check the dates stamped on the brake hoses too, anything 5 yrs or older has to be replaced without question.
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I would also change out the brake booster vacuum line... not the booster. The vacuum lne tends to break at the checkvalve.
I also would check the master cylinder after replacing the vacuum line. You might need a new master cylinder cap.
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