2016 magician rear brake issue

[Magician250]

Hey guys,first time posting here. My brother picked up a magician 250 enduro, everything seems to be in good working order expect the rear brake. We tried bleeding the brake system yesterday, it didn’t seem to have any air in the lines. Only fluid was coming out of the bleeder screw. The brake lever does not build any pressure, took the top cover of master cylinder off, it was full but the black rubber piece looked like it got pushed up from the bottom. I’m not sure what this means any help would be greatly appreciated. We have the brake pedal compressed down with a wrench hopefully will be able to find any air in the line. If this doesn’t work I am thinking either the brake needs to be adjusted or master cylinder needs to be replaced what do you think? Thank you

[JerryHawk250]

Make sure the plunger on the master cylinder fully returns when the pedal is released. In other words there should be a little play in the linkage between the pedal and plunger when released.