Bleeding the brakes on these trucks is an actual nightmare, here's a quick run down of some of the info I've seen on how to fix the issue you are having:
1. Make sure the Master Cylinder is bled correctly. It needs removed and needs to be bench bled until the two lines don't make any bubbles.
2. Reinstall the master cylinder and then bleed the connections to the two steel lines, but be careful not to pump the brakes during this step! Make sure the pedal is one being slowly pressed down while getting the line open (its recommended to start with the rear line). Make sure to close the line before pressure is release from the pedal.
3. Repeat #2 but now with the front line. Ensure that the pedal is not being pumped as it will not fix the problem you are having if it's being pumped
4. Bleed the system pretty much as usual from here. Start with the right rear, then left rear, then right front, then left front. Ensure the reservoir is not being emptied during this process.
If the pedal is still low then clamp the 3 flex lines and see if the pedal gets any higher. Whichever flex line has the most air will make the largest difference to where the pedal is positioned.
No promises that this will fix the issue but I've seen some people have success following this general process with these trucks, best of luck!