I take the same approach as Trev. Machine up a dummy axle (mandrel), set it up in lathe to run true, then fit drum onto axle, lock up with nut (friction lock), then machine true.
This is best done as an assembled wheel, as the spokes will sometimes pull the drum surface out of true (some designs distort less than others!). I can't fit a complete wheel in my lathe, so machine the drum before spoking. This has still givens a good result for my brakes.
So, once hub drum is true, then have oversize linings bonded (AND RIVETED) onto shoes, set up the backing plate up the same way, and machine away!
Use appropriate breathing apparatus AND vacuum cleaner to control / remove fibres during the machining. I like 0.010" clearance between drum and shoes. Also put 0.010" shim packer behind the flat portion of each shoe.
Brucie, what is also important is that the backing plate / shoe assembly is centralisedwhen assembled in relation to the wheel hub. You might get some improvement if you try this now.
Loosen axle a little, and torque arm, hold brake on, and pump forks several times. Whilst holding brake on, have assistant lock up axle, etc. Road test; you might get an improvement.
Richard