My neighbor asked me to check his tahoe out by driving it and telling him what I thought of the way the brakes felt. I picked it up to 30mph and stepped on the brakes. The truck stopped but the peddle was spongy and went to the bottom. I told him that a spongy peddle usually meant air in the system and that the peddle going all the way down was not good. He said that it had been like that for years, ever since he took it to the local chevy dealer for work on the brakes. So we took all wheels off and found many problems. One of the rotors was so bad it couldn't be turned, we replaced both along with new pads. The wheel cylinders were both leaking in the rear, replaced both. The shoes had a lot of meat but all were cracked down the middle, put new shoes. We then bled the system starting with RR wheel, then LR, RF, and finally LF, the peddle firmed up but if I held firm pressure on the peddle it would eventually go down. We re-bled system again and the peddle would still go down. We next replaced the master cylinder after bench bleeding and the peddle still goes down. I don't know what else to do. We did some research and found that this truck has a Kelsy Hayes EHCU and that this might need to be bled first then the rest of the system. We don't know what this is. Can anyone HELP US?