Okay so I have been messing with the rear brakes on a enduro dirtbike and I have wasted hours trying to get them bled and get the pads on. It has two "pistons" i guess, that push the caliper against the disk. When I press one down the other comes out and when I press them both down, the pedal stiffens up and as soon as I let off the clamp they go back to where they were. And they will only go out a little as I pump the brake pedal before they stop and dont want to go out any more. I can get the pads to fit on if I let out brake fluid from the bleed line, but them they wont go back out enough to push the disk!
What is going on? And is this maybe mean the caliper is bad or brake cylinder?
Any help greatly appreciated!

Thanks.