I am wondering if there are supposed to be two different sizes of vacuum line around the engine, because there already is in my car. I never noticed until one broke, that it is much larger than many others around it. And I didn't think it odd until going to order more line of that size, when all there is on every MB parts site [I know of] are the small 4mm size lines. I've got tons of this small diameter line, is it okay to use anywhere firewall forward?

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1981 300SD, odometer stopped at 188,000