[Bandit130]

I experienced similar braking on 72 C10 I purchased in 2014. Searched and found that flex (rubber) brake hoses fail internally. They actually close up internally over time not allowing proper fluid flow to defective side. It pulled very hard right, so left side was bad. Replaced both. Same thing happened on the 74 C10 purchased in 2015. PO had replaced left but not right. Had slight pull to left. Replaced right side. worked great, no more pull.

This may not be your problem. Wanted to pass on my experience.