thanks for the input guys but I'm scrapping that idea, anyway I bought myself a used brake booster(from another W116) for my '78 280S b/c the original one is now a junk,(hissing sound when applying the brakes and erratic idle even the carbs are good),before buying the booster, I checked the big yellow vacuum line for cracks and 1-way valve by sucking like other guys did in the forum, it was good, disconnected and checked the MC and the back side was dry meaning no leaks into the booster, I changed the big seal ring bet. the MC and booster w/ a VITON one but a little fatter than the original for better sealing, and lastly I had to change all brake hoses and rebuild both front calipers. The used booster is now installed and filled with my usual CASTROL DOT4 fluid and did a 1-man bleeding and start the engine and I got firmer brakes and lowered the idle a little bit and no hissing sound when applying the brakes. Whew, what a work in a day!

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