Recently on my 82' 300 sd the brakes require a lot of pressure on the pedal in order to operate effectiveley. I am wondering how would i go about determining whether this a result of the vacuum pump or the brake booster itself? I must also mention that this is only a problem when the engine is relativeley cold. After i drive for 4 or 5 minutes the problem goes away. My car also doesnt like to shut off when its cold, but equally this problem disapears as long as the engine warms up. The vacuum lines have been recently replaced and i checked them just yesterday and from my observation they seem to be fine. So what are the possibilities?

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1983 300SD White with Grey interior