Giant MPH 3 brakes (sticky piston) ???

cpk1

Hi, i have giant MPH 3 brakes on my bike, it seems to have a sticky piston, when i use the back brake it grips the disc but one of the pistons does not move retract back to the original positon it only moves back a smalll bit but is still binding on the disc. If i remove the caliper and and use a flat bladed screwdriver i can push it back in to enable the disc to move without contact. There are no leaks that i can see, Are the seals worn or maybe a bit of dirt stuck in the piston, hoses blocked, i dont know. Does anyone know the likely answer as i really dont want to have to strip it as i dont currently have a bleed kit to refill it. Thanks