Eli Shahar wrote [in 160614 MMDigest], "If your pouches are bad enough
that you can hear hissing leaks, or see a hole visibly, then a sealant
is not the solution.  Replace the pouch".

On the face of it you might think that was true, but in this case it
was not.  The pouch leather was relatively new and was in good order
with no holes, the hissing was the air travelling through the
connecting pipe from the accordion expression pneumatic (not a pouch)
to the valve, and the expression pneumatic in this instance was the
half inch one which works out at six square inches of pouch leather.

Six square inches of porous pouch leather can actually provide quite
a leak.  And I did have to stick my ear against it to hear the hiss
as I was looking for leaks.  Try it on yours, you might be surprised.

James Black, UK

 [ Lots of leakage is tolerable if not associated with a tiny bleed;
 [ for example, a vacuum reserve (accumulator).  But the air-motor
 [ and its knife-valve governor cannot tolerate any leakage at all
 [ else the motor speed regulation under load suffers.  -- Robbie