when water gets into these drum brakes, which it does, the pads will get glazed over and cause exactly what you're describing...pulling and jerking.
i once had the whole bike stand up on the one front wheel when it grabbed, going down hill...no clue how i saved that one
take the drums off and you'll see the glazing on the brake shoes, they'll look black and shiny.
sand them down until they look dull, and probably green pad material underneath
sand them evenly until all glazing is gone.
you can also sand the drum surface too, as it will get glazed over also.
do not use very rough sandpaper on either. medium to fine grit only.
reassemble,
then with the front tires off the ground, spin both wheels and hit the brakes, adjust each until hitting the brakes results in even stopping of the spinning tires. if when riding more braking is required, adjust each the same amount.
ps...this will only last until water gets into the drum and glazes the pads over again.
hence why hydro conversion is popular