As food for thought...
seeing the spring loaded pushrod tube referenced by Bela makes me wonder if
some kind of external spring could be installed over a tube (without
removing the tube) and press against some sort of seal applied linearly at
both ends; kind of like teflon string (as opposed to tape) that plumbers
use.

just changed the toilet inlet valve and saw the
shank washer just seal the valve body with finger tightness..

if not a spring, how about some sort of clamp over the tube that would hold
the hand fabbed string gasket tightly in place?

Since this is an experimental, what does everyone think??

thanks, Dick

http://www.realgaskets.com/files/horizontal.htm

It's part # RG-200PRT-1