Brake issue

[82chevdiesel]

Im having an issue with my brakes, when i first drive the truck after its been off for a little while (could be hours, days, doesn't matter) and Ii apply the brakes it seems as if the fronts are engaging before the rears. Im not sure if this is whats actually happening but it jerks the truck forward with only a slight amount of pedal pressure. After the brakes have been applied a few times they operate as normal. The brake setup is hydroboost with rear drums front disc. I was think maybe prop valve or air in the lines? What do you guys think?