That didn't really answer the question, which is WHY does the "cake"
form IN the filter media for vacuum fed but ON the surface for
pressure fed? I would think that, all else being equal, it would work
the other way because the pump is operating more efficiently with
pressure fed, so the velocity would be higher, driving the dirt deeper
in the media. Other than that, I don't understand how the felt media
knows whether the pressure differential across it is caused by
pressure on one side or vacuum on the other. Theoretically, it
shouldn't matter. So I'm just trying to understand the physical "real
world" process that causes it not to behave according to theory.

Steve

Possibly the Vac system is more constant, whereas the pumped system
pulsates, this may affect the level of turbulence causing the crap to
lie long ways across the filter, but with a more smooth flow (vac
system)the crap will align with the flow and penetrate deeper into the
filter media.

This is a blind guess, but may prompt someone who knows about these things.