If your brackets, callipers and rotors are from CPP, should be no flex. Pads only relax not retract. If you can pump up the brakes, that does usually mean air in system. Be sure to examine, with a good light, all areas of truck for leaks. The leak does not always drip to floor or is not easy to find. A fluid leak is also an air-in leak. Could be at any fitting joint/hose joint/banjo. What exactly is flexing and what direction? What truck is this? Good that you bench bled MC. After no leaks found, Bleed and bleed again. Some bleed methods don't work so good by yourself. The residual valves are necessary because the shoes do retract allot because of the big springs. They get too far away from drum and the Res Valve keeps the pressure "on" them. Be sure you have those in the correct port(s). If residual valves are present and in correct place, I say it's air in system or leak and needs more bleeding. You are pumping up the fluid AND the air to get stopped and when no pedal pressure, system looses fluid volume sort of. If you don't pump, does it go to floor or just near floor. Like you said you did, recheck the shoe adjustment again also, drum should stop turning then back off til starts to move....barely.
