I know this is a sec gen truck, but i only post here those sec gen guys are mean

We have a one ton dually 95 model and the brakes are worthless,pedal goes to the floor and if you hit a bump they lose their back brakes,,it has a new,,master cylinder,,brake booster,,proportional valve ,,real wheel cylinders,,antilock unit,,both pads and rear shoes are new and adjusted according to the book.We have bled the wheels the antilock the proprtional valve, yet still no brakes. So any ideas on why theres no pedal and it wont stop ?

 

I wonder if the rubber lines are delaminating from the inside. I have a similar issue. Replaced the outer lines a couple years ago, but left the inner. Everything else has been replaced, but I still need to pump the pedal a little. I have new calipers too.
Don

 

I've heard about that too. The hoses will look fine from the outside but they are collapsed on the inside... it's worth a check