brake pedal sticking????

Ok guys i tried the pedal while the car was off and no sticking pedal it only does it while she's running my fluid is only about three months old the pads and shoes and drum hardware are all 3 months old as well its not like the brakes are sezieing up its just like the pedal is sticking if i pull forward on the pedal it goes back into the correct place but when i step on the pedal it feels like it pops into place than when i let off it stays engaged untill i lift up on the pedal with my foot. Im gonna pull the master cylinder and try to bench bleed it to see if the plunger spring is to blame if not i bet it's the booster...