John Grant was right in describing problems and solutions.  I have
a few more potential issues.  If Alexander is lucky, the valve stem may
be threaded, allowing infinite, delicate adjustments to be made at the
bench.  I agree that there needs to be "wobble".  I am doing that as we
speak.  Most Standards, however, are of the pressed fit collar variety.
You may need to create a jig to set the position before you install the
valves.

There can be an issue at the pouch end of the valve stem.  If it is
set too deep in the pouch well, the pouch itself may lift the valve up,
allowing leakage.  Since best repetition means minimal clearance here,
if the pouch has shrunk over time or has stiffened, or if the pouches
were replaced with improper dishing (or sometimes just looking at it
cross eyed) the valves leak.

How to bench test this issue?  Lay the valve chamber without the pouch
board attached but with all the valves in place.  Support it off the
bench, face down.  Gently press down on each valve.  If it is firmly
and fully touching the inner valve seat, there will be no movement.
If there is a subtle "bounce" there is a problem: the valve leather
is slightly warped or the valve itself is not able to do its wobble
thing.  Solve the problem.

Next, attach the pouch board.  Again, check each valve for bounce.
If there is some now where there was none in the previous test, the
issue is probably where the valve button meets the pouch.  It is up
to you to solve the problem.  Is the button going in too far?  Is the
problem in the pouches?

Other more peripheral issues can also be a problem.  Has all the gasket
leather been replaced?  Even the gasket leather where the pneumatic
decks are screwed to the valve chamber may make a difference.  Old but
still good looking leather may allow air to seep through to screw holes
and into the stack.  Do all the screws tighten up snug?  If not, size
the holes as necessary.

Good luck,
Jeff Davis