Howard Wyman asks two questions in Friday's digest.
[ Matthew's other answer is in a separate article.  -- Robbie]

First the easy one:  vacuum pumps need no stiffeners in them.

Pressure pumps need them to keep the pressure from pushing the
leather outwards, deforming the pump shape, on the compression cycle
and snapping the leather back inwards on the intake cycle -- a sequence
of events which would severely limit the life of the pump and its
ability to deliver maximum air.

Vacuum pumps do not seem to demonstrate any tendency to push their
leather sides outwards on the exhaust cycle, so they don't need
stiffeners.

Matthew Caulfield