Dada1998

I have a 97 GMC Sierra 1500 and I got it with the brakes bad but I’ve replaced the master cylinder brand new the wheel cylinders in the back are both brand new pads are brand new brake booster is used but I know it works still. I’ve messed with it for days and can’t figure out what it could be! Does anyone have any ideas?My check emergency brake lights comes on and off when applying pressure to the brakes. I have brakes after I apply to the brake about 80% down to the floor but it barley wants to stop. I need suggestions please