If you haven't replaced any hoses I would look there next. Especially the rear one as it holds fluid for both rear wheel brakes.
They swell on the inside and restrict any volume from reaching the wheel cylinders.
Adjustment is key to keeping drum brakes working well.
On the rear of mine I usually set the LR until it will hardly spin by hand and then set the RR creeping up on it till I can hardly turn the wheel.