Brakes lock up

[Timberman]

I own a 2004 silverado 2500HD with the Duramax motor. Recently I replaced the brake lines, master cylinder, and hydroboost. After bleeding the brakes we were ready for a test drive but when the motor was started the brake pedal went down and the brakes were completely locked up. When the motor was shut off the brakes released. Thinking that this had to be a hydroboost problem, I exchanged the hydroboost for another one. We installed the new hydroboost and bled the system and tried again. Again the brakes immediately lock up when the motor is started. I am all out of ideas as to why this is happening. Any help would be appreciated.

[davester]

maybe unbolt the hydroboost from the master cylinder and see what happens?

 

there is a rod that goes from the hydroboost to the master cylinder (IIRC) and there are rods of two different lengths depending on the vehicle, maybe you have the rod that is the wrong length for it? Or the rod isn't properly seated in the master cylinder? Or maybe the same for the rod from the brake pedal to the hydroboost?

[C/K Man]

Are you sure you put the lines on correctly?

[big white 3500 denial and]

I own a 2004 silverado 2500HD with the Duramax motor. Recently I replaced the brake lines, master cylinder, and hydroboost. After bleeding the brakes we were ready for a test drive but when the motor was started the brake pedal went down and the brakes were completely locked up. When the motor was shut off the brakes released. Thinking that this had to be a hydroboost problem, I exchanged the hydroboost for another one. We installed the new hydroboost and bled the system and tried again. Again the brakes immediately lock up when the motor is started. I am all out of ideas as to why this is happening. Any help would be appreciated.

I d start with the hydro boost lines, sounds like you may have swapped a line ?