I had the same problem on my right front on my '46 several years ago. When I couldn't figure it out, I bit the bullet and pulled the wheel cylinder so I could inspect the parts. I found a bit of rust in the center part of the bore. Used a hone on it, checked for free movement of the pistons and rubber seals, reassembled and bled the entire brake system. Solved the problem completely. In my opinion, sometimes you just have to disassemble and inspect the parts.
I like Paul's idea: pull the drum and have someone very gently depress the brake to see if the wheel cylinder is moving both pistons. Don't push too hard or you'll blow the pistons right out of the wheel cylinder!
