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The most common reason vacuum pumps fail is heat. RAM made some shrouds to force cooling air across the vacuum pump during flight. By the way, Richard Colins of FLying magazine has a sotry abotu how his P210 went through vacuum pumps at an amazing rate until he installed the cooling shroud. This colved his problem.
A test you might run ...
On the ground, when you start only the engine with the "Bad" vacuum, at something above 1200 RPM, does the vacuum gage show the needle well into the green arc?
Repeat that test using only the engine with a good pump - and is there a difference?
The vacuum manifold has come adjustment on it to set the pressure it's trying to regulate - that might be worth a look.
Of course, leaks in the vacuum lines will certainly make the pump work hard - and possibly exceed it's ability to achieve the desired vacuum pressure.
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