[alsriv2]

Did you completely bleed (bench bleed) the master cylinder before you installed it in your truck? The process is easy to do and easy to screw up if not paying attention to the air bubbles.
I recently messed up my bench bleed on my new master cylinder and I spent hours puzzled why my rear brakes were not bleeding. For me it was caused by poor fitting plastic threaded nipples used in the bench B process. The nipples were allowing air to be sucked into the reservoir and I had to replace the cheap plastic ones with steel ones.
2nd BB with non-leaking lines solved the problem completely.