It sounds like your Seeburg L has the winter syndrome (too dry).
Place a hygrometer inside of it and take a reading.  I keep mine
at 35% relative humidity.  I do not know what leaks, but it happens
during the winter heating season.  The addition of moisture usually
solves the problem.  I keep four containers of water in the bottom
of the Seeburg, and if the hygrometer reads below 35%, I add damp
towels and hand them on the inside.

In my Seeburg L the stack vacuum is 20" for normal playing, but
when the soft "choke" is engaged by the roll the vacuum drops to 10".
The vacuum can be regulated by a screw inside of the vacuum reservoir.
The soft choke is regulated by a screw on the front of the small
pneumatic on the front of the distribution box.

Before you make any regulations, increase the interior humidity for
a few days, and see if there is an improvement.  Perhaps someone might
have more knowledge of this problem.  I know of others with the same
problem.

Bruce Clark