Dave,  I just finished rebuilding the bellows and reservoir (1) for
a Cable Euphona.  I get about 15-20 seconds of time too.

In tracking down the problem, I found leakage where the pushrods that
control the valves go into the 2 boxes that mount on the top of the
'manifold'.  I believe that one box is the governor, the other is the
rewind motor valve.  Through normal wear, the hole has gotten larger
than the rod.

Other than that, I suspect there is very minor leakage through the new
rubberized cloth and maybe even the wood itself.  Minimal as the
leakage is, per square inch, I suspect that maybe it all adds up.

To fix the pushrods, I decided not to rebuild the end of the valve box
and governor, but I put a little bit of silicone sealant around the
shaft and slid it back and forth through the hole, then let it set up
for a while, sliding it in and out every so often while it set up so
that it didn't ever lock the shaft in place.  I think this reduced the
hole size a little bit.  Other options might be to glue on a rubber
seal with a hole that matches the rod diameter.

As to the rubber and wood leakage, I thought about coating the wood
with some sort of sealant, but I just repainted it instead.

I look forward to hearing about other people's sources of, and fixes
for, leaks.

How much leakage is acceptable, and how much is to be expected?

Bill Mackin