A rare condition I experienced once, was from mistakenly/wrongly using transmission fluid instead of brake fluid. Caused locking and hyper sensitive braking..

 

Hey rovers, signing back on to report on the solution for the problem stated in the original post. The truck needed a new brake booster - first hint early on was that my brake lights were on all the time because pedal wasn't reaching the brake light switch, I thought maybe some kind of rubber grommet or something may have fallen off. ultimately it seems my brake booster failed in such a way that it would never fully retract off the master cylinder - this made my old original master cylinder fail so I had to keep the new master cylinder and get a new booster. After fitting the new booster everything works like a charm and my brake light switch is working normally again. I am amazed that the brake booster failed in this way, every testimony I've read of failed booster usually involved no power brake assist. Thank you to everyone that chimed in to give me ideas !

 

@kalishnakov_rover Thanks for reporting back, that is weird for sure