hey TH- I'm actually going through the same thing on my bn4. Not sure what model you have, but I've found the drum brake cars to be especially frustrating to remove all the air from the lines. Bench bleeding the master is a good start. I assume you've checked all connections for leaks? Also, I make sure I have the brake shoes (on my bn4) adjusted so there's very slight drag on the drums (to minimize travel of the brake cylinders). I always start from the furthest brake cylinder from the master cylinder, affix a tight-fitting line to the end of the bleeder nipple down into a Heineken bottle with the end in clear brake fluid. Then have my very patient helper pump (hard) the brakes multiple times, hold down while I crack open the bleeder and watch the bubbles flow. I do this multiple times, each time checking that the MC is refilled (since you'll lose a lot in the process into the beer bottle). I then move around the car eventually ending up at the driver's side wheel. The factory manual says to open the bleeder up while operating the pedal with a "slow full stroke until the fluid entering the jar is completely free of air bubbles. Then, during a down stock of the brake pedal, tighten the bleed screw sufficiently....". But I haven't found that method to work for me. I've also heard of other folks isolating the wheel cylinders after they've bled them by clamping the rubber brake line carefully with hose pliers and then continuing around to the remaining wheel cylinders. That may be my next plan of attack. I guess my response to pumping brakes is different than the other folks who've chimed in- I've found it habit (over the 20 years I've driven my bn4) to pump my brakes twice each time I use them (except when panic-stops don't allow this!).
Good luck-
Austin