In rec.aviation.piloting Thomas Borchert wrote:
Mxsmanic,

Why not use manifold vacuum as the primary source, instead of a separate
vacuum pump that provides an additional point of failure?

Because Manifold Pressure sucks (and that is a serious and true answer).

The manifold vacuum
would be there as long as the engine were running, without the need to rely on
a pump.

Wrongo!

This was written by someone obviously too young to have ever driven a
car with manifold vacuum powered windshield wipers and little to no
understanding of real engines.

--
Jim Pennino

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