if you are basically saying that pumping lever a couple of times makes brakes work better then it is prob air trapped / seals in lever damaged or worn so they are not pushing fluid correctly to activate pistons. non returning pistons can also be air trapped as there will be less vacuum effect to pull back pistons.
one test is to take wheel out put large screwdriver between pads pull lever and hold. if you can push pads back with screwdriver to any extent while holding brake on then prob air.
make sure you use fluid that isnt too old and should be left to stand for a few hours so no air bubbles in it.