Hy rustyoldtruck, I have allways bled the primary system first [the one closest to the brake pedal], I have also used the primary system to activate the largest brakes, on a half ton it doesn't make much difference. After the primary is bled you can turn your attention to the secondary, once the primary is bled it will shorten your pedal stroke considerably, what I have done with success in the past is to leave the last bled primary bleed screw a little loose and immerse the bleed hose in a jar/bottle with a little fluid in the bottom, this will allow you to get a better bleed stroke with the secondary system. Keep checking your master cylinder level and the level in the primary system bleed jar/bottle, hope that helps.
