Cherokee Brake issues......?

Ok, I am having some very frustrating problems with my Jeep. It has had brake issues since I replaced the front end and drained all the fluid from the master cylinder. I have replaced both front calipers (I couldn't bleed one side) and the master cylinder, but it still only stop marginally (that means over 20-30 mph you won't...at least it will take way too long). The pedal travels all the way to the floor still. I have not been able to bleed the rear brakes at all and I need to know if this might be my problem. The last time I replaced the master cylinder I was unable to bleed the rear brakes and it stopped fine....not great mind you, but I was very driveable. Oh, the reason I have not bled the rears is beacuse the bleeder screw is rusted tight on both sides. So do I need to replace the wheel cylinders or do I have a pressure leak somewhere? Please help! I have already spent $150 on parts that I thought would fix it! NO SMART ES!!!!!!