Brake pedal goes to floor

02 chev 1500 4wd 45000mi Recently was told the rear pads were needed to be replaced. Once I started to do the job myself I noticed that the left side was at completely worn but the right side was almost perfect. I thought this was strange and the truck had been in a accident on the left side. I forced the piston back very slowly with a c-clamp and changed the pads. Once I was done the the pedal felt fine until you start the truck then it stops but has excessive travel to the floor. I have bleed the system still can't fix it. I have done this job many times in the same manner on many vehicles never had this problem. I changed the caliper on the side with the excessive wear still no help. Please help