Installed new rotors, pads, and shoes on the truck a few weeks ago. I decided to bleed the brakes by myself and it seemed as there was air in the system. Repeated the process twice with a buddy and the pedal still goes to the floor.

While the truck is off, I can pump the brakes till they are firm. As soon as I start the truck, the pedel goes to the floor. My buddie thinks it's the brake booster so I thought I would double check before I headed to NAPA.

 

I had the same problem with mine when the garage I went to did not change the washer/seal that connected the brake line and the caliper....look around to see if there is any leaks anywhere in the line.

 

When I was under the truck I didn't see any leaks comming from the caliper area. My logic (feel free to correct me) is that the lines are fine due to the stiffness of the break pedel before start up. Since the problem occures when the truck starts, I'm looking towards the break booster area. I will pull the booster line off tomorrow to make sure there is a vaccum (sp).

 

When you bleed the brake lines make sure you start at the right rear wheel then left rear, right front and finally left front. The theory is you should start farthest away from the master cylinder because instead of eliminating any air you could just be shifting it around the lines.

 

Yup, read the book before I started. I'm sure there is no air in the lines. We went through a lot of brake fluid and never let the master cylinder run dry.

 

Ill bet one of your calipers isn't sliding like it should. Or check the clamps on each of the rubber lines by the calipers. You sometimes need to pry the clamp open and work the line in your hands to free it up. What happens is rust will build up under the clamp and put such a tight squeeze on the line either one of two thing happen. The caliper will freeze in an open or shut position......Rob

 

If you have to pump up the brakes there is air in the system. When you are pumping with the engine off you use up the reserve power assist (vacuum). When the engine starts the power assist is renewed. Try letting them gravity bleed for a while.
Good luck and let us know how you make out.