GMC Repair: 1999 GMC Sierra Classic Brakes, gmc sierra, rear brakes

Question
I have a  very spongey brake pedal, I have replaced rear wheel cylinders, master cylinder,fluid and one caliper. have bleed the system numerous times. even bleed the abs unit.the rear brakes were hard to bleed , not much fluid would come out either side. the truck stops OK, but pedal is spongey.  

Answer
First try to open the wheel cylinder bleeder screws and just let it sit. Gravity should bleed them and eventually fluid will drip freely, if not then there is a restriction somewhere. Also take off the master cylinder and feel inside the booster for brake fluid. If there is fluid inside the booster it needs to be replaced and is probably the cause of your problem. Also you can try to bleed the master cylinder on the truck by loosening one of the lines and the having someone else slowly push the pedal to the floor. Once the pedal is down tighten the line again, if there is air present then continue until there isn't. Do this for both of the lines. If you do this the pedal may stay down, just pull it up.