Eliminate the transmission. I didn't know it was a manual.

Brake fluid can be pulled through the vacuum diaphragm if you have a leaky master cylinder. The surest way to check for a brake fluid leak from the master cylinder is to slide it forward on the two mounting studs and look for moisture. Many times you will notice a trail on the outside of the vacuum booster but this can be hard to spot with how they are mounted on the W114's/115's.

I had a BMW 2002 which only smoked when doing a hard left turn but when it did it laid out a smoke screen. Took me a month to figure it out. The leak was very small and until the fluid accumulated in the booster it would never smoke. Hard left the fluid would slosh over to the vacuum line and then, instant cloud!

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now