Standard Pneumatic Co. actions have a couple of idiosyncrasies.
The most significant is the secondary valves: they get stiff and
then don't seal.

You need to remove all of them and then clean out the gunk that is
between the plates and the leather.  I use a very thin burn-in knife.
A dull razor is also thin enough.  You need to get to the point that
the leather can wobble on the stem.  The valve can then slide as it
seats.  Don't forget to clean the valve seats, as well, and be careful
when you reseal everything.

The bellows are usually easy to work on.  Unlike many other systems,
all the parts are screwed together and have leather gaskets.  Use
a stethoscope to listen for direct leaks.  I just purchased one at an
auto parts store for about $20.

Try using incense for finding leaks that seep.  By watching the smoke,
a leak will show because the smoke no longer wafts upward, but goes
into the leak.  Also, as you approach the leak, the tip of the incense
may glow brighter as more air moves over it.  I get stick incense at
the local grocery store.

Take a look at the tracker tubing.  If it still has the original lead
tubing, you may consider replacing it, because it tends to oxidize and
develop holes.  If there are bumps that look like warts, especially
around the bends, then it is about time to replace it.

Jeff Davis in Seattle