Brakes on a 1990 F-350 4x4 7.3 diesel. I have replaced the whole system..

What is your question?

Meaning the back drums and front calibers, I also have replaced the booster, master cylinder, & the ABS block on the frame for the back brakes only. I used the Haynes auto manual & procedures to bleed the system properly as well as installation. I have got a good petal to start with but it will leak down if I hold it down for a short period of time so I'm totally confused on what is going on here any suggestions will be greatly appreciate.