People have their favorite methods such as chilling the box in the
freezer or heating it in a microwave.  If the box is original, the glue
is likely to be weak enough that either method, or just using care with
a long flat knife, will work fine.  Sand the box to expose the glue
line and carefully split with a knife.

You do not want to use a knife or chisel with a blunt taper as this is
more likely to wander off the glue line.  If the knife seems to be
going into the wood, try starting again from a different position.
Some people like to position the knife and give it a sharp whack, in
hopes of splitting it with the shock.  Others like to go slowly.  In
the chilling method, the sharp whack is used and a knife may not even
be needed.

Another technique is to saw the box apart with a fine-kerf blade.  This
is the safest method, but it will remove a little material and change
the alignment of the components a little.  This is not a problem if a
gasket is used in reassembly.

Before cutting or splitting, be sure to check for screws holding the
box halves together.  Some boxes were screwed and others were not.  Be
sure to measure the total height of the box before splitting, so you
can get it back to the same dimension, if you saw and or repair it.

If the box is original be sure to note and mark the positions of all
linkages.  It's a good idea to photograph it as well.  The position of
the exterior knife-valve operating lever with respect to the knife
valve determines the expression curve and must be maintained.  This is
not the same as the adjustment you can make with the 0 setting screw.
When you reassemble the box, the lever should be in the same position
it was in before disassembling.

I have used different methods and don't really have a preference,
although I would probably use the saw method if I weren't too cheap to
buy a Freud thin-kerf blade for $80.

If you do cut the box, do it in multiple passes on a good table saw and
make sure the box is square and smooth before cutting.  Of course, if
you have a high quality band saw that is big enough, this would be even
better.  Start by going around the whole box at a shallow depth.
Increase the cut depth at each pass.  Fine-kerf blades can wobble,
especially if they overheat.  There may be a small remainder inside the
box that is not cut.  This can be cut with a hand-held hacksaw blade.
Most of the box is empty, so there is only going to be a little
remainder, if any at all.

I highly recommend screwing the box together and not gluing it.  It
will make it much easier for the next person who has to open it, and
this may be you.  Whether you cut the box apart or split it, it is
likely that there will be a little loss of wood which will effect
dimensions especially the valve travel.  Sand the halves flat before
reassembling.  If a lot of damage was done, you will need to repair the
wood.  If you are not gluing it together you can use any reliable 
patching method; but if you are going to glue it with hide glue, it is
best to glue in wooden repairs with a permanent glue such as epoxy,
since the hide glue may not stick well to things like Bondo or other
fillers.  You can apply a layer of thick veneer or very thin plywood
(available from hobby shops), if you need to restore the entire
surface.  Make rough holes to match the box channels and then trim
afterward with a sharp knife, die grinder, or veneer saw.

My favorite material for a gasket is polyethylene sponge, which is
available both with and without pressure-sensitive adhesive.  Do not 
use urethane sponge, unless it is of specified quality and guaranteed
to last for fifty or more years.  Most urethane will turn to goo in
just a few years, whereas polyethylene will last for 500 years.
Polyethylene is easily compressed; so you should be able to achieve the
correct valve gap and original box height by adjusting the screws
progressively.  Use a large number of screws carefully placed through
all the full divisions, and be sure to not do something really stupid
such as driving them through the pouches.  Before you drill the holes,
consider where the screws will penetrate.  If you don't have a drill
press to drill straight through the divisions, it is probably best to
not use this method.

If you use polyethylene sponge, it is a good idea to place a few wooden
spacers in the gasket to make sure you don't compress the gasket too
much.  You could also get fancy and use flat-head screws and adjust to
exactly the correct spacing.  Be sure that the gasket surrounds any
spacers so you don't have any leaks.

One of the most critical and difficult aspects of rebuilding the
expression regulator is getting the knife valves to operate properly.
Both the stationary port block and the moving knife need to be made
absolutely flat, lubricated with graphite rubbed in with alcohol, and
smoothed with very fine sandpaper.  The linkages must not bind nor the
knives lift off the surface of the port block with any combination of
front and back knife end positions.  The knife moves a lot at both ends
and it needs to still seal at the port in any of these positions.

If you like to see things in action and want to verify that the knife
valves are working properly while playing, you can use thin poly-
carbonate to cover the regulator pneumatics.  Do not use thick material
because it will add weight and inertia to the boards.  Do not use
acrylic as it will crack in time.  Use plenty of small screws and a
reliable gasket such as polyethylene sponge.

Spencer Chase
Garberville, CA 
http://www.spencerserolls.com