James Waldby fired this volley in news:E4-
:

If the push rod and vacuum inlet had their positions swapped,
pulling a vacuum would pull the membrane out against the cylinder,
improving the seal. Conversely, pneumatic operation would tend
to collapse the membrane.

In short, from your earlier description I had envisioned the
membrane being on the other side of the vacuum, ie, with the
vacuum inlet on the left and push rod entering on the right.
So, have I got things backwards, or maybe the direction doesn't
matter?

Y'know, I might have mis-remembered the arrangement -- it was, like,
1978-to-1982 when I built it, and I was copying another device I
couldn't afford to buy.

I think you're right about which end the vacuum goes on. I didn't
catch that when I drew it, but it would make more sense.

LLoyd
(Chet? You listening?)