I have on occasion worked on trucks with basically the same issue you describe. Most of the time it was one of two things either stuck pistons in the cylinders which you have addressed, or brakes far enough out of adjustment that the pistons were at the end of travel and shoes were still not in contact with the drums. I would check brake adjustment first and see what happens. International Harvester truck division said to adjust brakes until drum was locked tight and then back off 3 clicks on used lining and 5 on new. As I recall if they are that far out of adjustment the first application of the brake goes pretty much to the floor and the second come up with a hard pedal and no brake. You do not mention that as a symptom so this may not be what is going on, still worth knowing though.