Because gasoline powered engines that run a vacuum in the manifold have a far more "abundant" source of vacuum. If you have a huge vacuum "pump" in the form of a naturally aspirated gasoline engine that naturally produces vacuum in a "larger" reservoir (the engine, basically) than the little bitty pump we have, then you can stomp the pedal all day and as long as the engine is running, you probably won't deplete the supply. Or to put it another way, the vacuum "consumed" by one press of the brakes is replenished almost instantaneously.

Our diesels don't produce manifold vacuum. Usually, they run positive pressure because of the turbo. Non-turbo diesels still don't use a throttle, so they don't produce vacuum in the same way a NA gas engine would. So, we have to have an auxiliary pump. This pump isn't as effective as we're "accustomed" to on our gassers that never suffer from hard brakes, and so when you deplete the entire vacuum system with 5 presses of the brake pedal, it takes a noticeable length of time to replenish the supply for vacuum assistance.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL