2000 Sierra brake issues

[My2gmcsierra]

Hi guys, was hoping you might be able to help. I recently replaced brake lines and master cylinder on my 2000 z71. Bleed the brakes, used the abs code reader to take care of the abs box, however, breaks felt great until I started the truck, when all resistance disappeared. Thought it was the vacuum lines but good vacuum. It's like the brake booster isn't there. Listened for holes in the lines, couldn't hear any...

 

Ideas. Thoughts? Did I miss some steps?

[O_J_Simpson]

Bleed and check for more air.

[My2gmcsierra]

Done and done OJ....this is a vacuum/boost issue problem...no air in system as everything's fine until the truck starts. Then all pedal pressure is lost. Checked the booster. It's fine, no blown diaphragms ETC. What am I missing?

[jrow8162]

Have you pulled the vacuum line from the booster after starting the truck? Can you hear it releasing the pressure when you do? Sounds like an issue with your booster. That or you still have an air pocket somewhere in the system. Did you bench bleed the master before you put it in?

[My2gmcsierra]

Checked vacuum and booster. All good. Master bench bled before install.

[dino688]

Ok so lemme put this one out there. Did you disconnect any of the calipers? I had one seize up a month ago. Bought brand new ones and I accidentally put the drivers side on the passenger and vice versa. Make sure the bleeder valve is on top! I replaced my master cylinder thinking that was it until someone a hair smarter than me noticed i installed them on the wrong sides.

 

Also make sure that you're bleeding from the farthest brake first so rear passenger then rear driver, etc.