Of course clean the filter.  The other thing I do is every once in
a while (each year or 3) is to screw off the cap at the handle end
and pull out the rod and piston.  I work a little light oil into the
leather cup "sucker washer" and spread it out some so that it presses
against the inside of the barrel.  I can usually tell if it needs this
by holding my finger over the inlet and pulling the handle.  If there
is not significant resistance I service the pump.

John Dewey

 [ Hydrophane leather dressing might be applied instead of
 [ light oil to lubricate the seal.  See the articles indexed at
 [ http://www.mmdigest.com/archives/KWIC/H/hydrophane.html 
 [  -- Robbie