Hy coeleo, I must admit I can't recall ever seeing a brake adjuster on the rear of any truck I have ever worked on that was a tiny 5/16" hex bolt. The older adjusters at the top of the backing plate were 5/8" hex if I'm remembering correctly and the newer brakes had two slots in the backing plate, I'll do some research but a picture sure would help. Do you think anyone might know what rear axle has been swapped into this truck?