Having just successfully rebuild a wind motor, I can offer the
following guidelines.  Assuming that the bellows cloth is good,
yes, making sure the surfaces are flat is important.  I graphite
the entire surface of the main body (soft pencil works fine here)
and then sand the top surface by hand using about 180 grit paper
on a very flat surface.

Once all of the graphite is gone, you know you have a flat surface.
I repeat one more time with 220 or higher grit just to get a very
smooth surface.  I do the same for each of the slides.  Sand slowly,
removing as little wood as possible.  Then apply graphite liberally
to each surface.  I suspend mine in a little alcohol or acetone
(I don't remember which) and brush it on.  Now reassemble.

Also important are the two metal rails that holds each of the slides
in place.  They should be adjusted so that the slides ride flat on
the surface, but not so tight as to bind in any way.  In other words,
the rails are simply guides and should not exert any pressure on the
slides.

Look at each slide from the side.  The guide should be in the middle
of the slot and you should not see an air gap between the slide and
main body.

Next adjust the "timing" for each slide.  When the slide is pulled
to its maximum open position by the crankshaft, it should just fully
uncover the hole in the block.  Verify that when in it's fully closed
position that the hole is fully covered.  This may need a little
tweaking for maximum power.  No two pneumatic ports should be fully
covered at any time.  Good luck.

Jim Quashnock