How to push in the piston even if the bleeder is open it still wont go back in?

open the bleeder valve

I'm assuming your talking about a disk brake, you need to use a C clamp and the old brake pads to force the piston back into the caliper. If it had a X in the head of the piston you need to get the tool to turn it back in. If all that doesn't work, the caliper is frozen and it needs to be replaced