Chevrolet Repair: brakes, wheel cylinder, rear wheels

Question
i have a 1984 1/2 ton automatic 2 wheel drive 350 engine i put on a brake master cyclinder bench bled it and then tried to bleed the brakes.right rear to left front had a good pedal went to move truck and the pedal was spongy.bled the brakes again and it seems i had a lot of air in the rear wheels did it again with no air coming out  went to move truck and had a spongy pedal. thought i had a bad master and exchanged it with the same results,checked the brakes and had good meat on shoes and pads.thought i probaly had opened up the bleed system too much so i did it by cracking the bleed just a hair till no air came out and a good pedal went to move truck and had a spongy pedal. talked with some mechanics and they said the the brake master would eventually bleed as many times as i have bled the system.i have bled this system many times with the same results it seem the left rear wheel has a lot of air in the system when i bleed and after i go to move truck i will get a spongy pedal and i then go to the rear wheels where i get a lot of air the fronts seem to be good as i don't get any air after bleeding the system.is there something else i can try or am i doing this all wrong? never had this problem before.

Answer
JIM,WITHOUT SEEING THIS,,,THIS IS JUST A GUESS,YOU SAY YOU BLEED THIS TRUCK AND ALL THE AIR IS OUT UNTILL YOU START THE TRUCK ??? WITH A PROBLEM LIKE THIS THE ONLY WAY TO FIND THIS IS GOING TO BE TO BLOCK OFF THE PORTS AT THE MASTER CYLINDER...BY THAT I MEAN PLUG OFF BOTH LINES AT THE MASTER CYLINDER WITH SOLID PLUGS. HIT THE BRAKE PEDAL WITH CAR RUNNING, IT SHOULD BE VERY HIGH AND HARD. IF NOT THERE IS STILL AIR IN THE MASTER CYLINDER..REMOVE AND BENCH BLEED AGAIN.THESE MASTER CYLINDERS WILL TRAP AIR BECAUSE OF THE TILT THEY HAVE. IF PEDAL IS OK AT THIS POINT YOU HAVE PROBLEMS DOWN FROM THERE, SUCH AS A LEAK IN A LINE A FAULTY BRAKE HOSE, WHEEL CYLINDER. YOU CAN ALSO USE A VISE GRIP ON ALL THE BRAKE HOSES TO STOP THE FLOW OF FLUID TO THE WHEELS.[NOT A GOOD PRACTICE BUT IT WORKS] START THE CAR AND AGAIN THE PEDAL SHOULD BE HIGH AND HARD,IF THE BRAKES ARE GOOD REMOVE ONE VISE GRIP AT A TIME AND TRY HITTING THE BRAKE PEDAL AGAIN UNTILL YOU FIND WHICH WHEEL MAKES THE PEDAL SINK..THEN FOLLOW THAT LINE AND BRAKE SYSTEM, DRUM, CALIPER WHAT EVER.IF THIS TRUCK HAD ABS I WOULD SAY THE PROBLEM IS IN THE ABS UNIT..BUT THE TRUCK IS TO OLD FOR THAT.. HOPE THIS HELPS,TOM.