My guess is the gasket, also.  I have replaced pouches with leather
and never had a problem.  I seal them with rubber cement twice and rub
baby powder into the pouches.  Open each tracker bar hole one hole at
a time and check if valve makes a sound.  Some would call it a spitting
sound or hiss.  They have too much travel if they make a sound.

If any of the valves don't work then you need to see if there is any
blockage between tracker bar and pouch.  You should be able to blow
into the tracker bar hole with your month using a piece of tracker bar
tubing and tell if the valves are working with no vacuum going to
stack.

If all tracker bar holes are covered then measure vacuum going to
stack then measure vacuum at tracker bar hole.  There should be no
difference but in the real world with thirty inches suction before the
stack, and then with stack in-line, there can be a drop of an inch for
the stack to still work.

A vacuum cleaner type turbine pump is not the best source.  An old
Ampico type box pump is handy at this instance.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana