The U-shaped saddle bolts that hold the stack in place also hold it
together.  If the stack is loose, considerable leakage can take place
at the pneumatic joints between tiers.  Also, the stack can move around
when loose, so it must be in proper alignment with the keys before
tightening.  This, however, is more of a winter problem than a summer
problem.

The earlier sponge neoprene valves were glued to their plastic
"spinners", and the original adhesive often failed within only a year
or so.  A different adhesive was tried, which began to fail after about
forty years, or sooner if improperly applied.  Once unglued, the sponge
neoprene punching would move to one side, allowing air to rush by.

Replacing the valves with the later type, held together with brads and
washers, will (if still available) bring the piano back to its original
poor performance standard, as long as everything else is okay.  It is not
necessary to break anything apart in order to do this.  The old valves
and spinners are pulled out with tweezers through the top opening,
tapping out any neoprene the stack may have swallowed.  New valves can
then easily be squeezed and popped in through the same opening.

When the plastic unit valves were introduced around 1960, a very few
stacks were built before the switch to sponge neoprene; these worked a
lot better and were about as responsive as the 1957 Hardman Duo stacks
built the old-fashioned way.  Those pianos when new were actually fun
to pump, and the electric motor was available only as an option.

To make the later instruments play "better than new", however, will
require replacing all the plastic unit valves with ones of an improved
design.

Jeffrey R. Wood