Look on the sticker in the glovebox. You've most-likely got JB5. The little JN3 brakes were discontinued a few years before your truck was made. Those had a 1" bore master, with a 34mm or 36mm QTU at the back, a smaller booster, smaller pistons in the front calipers, and thinner rotors.
If you have JB5, you had a 1 1/8 bore master with the 40mm QTU.
JB6 was the same master and booster, same front brakes as JB5, but with 11.x" Duo-Servo drums in the rear instead of the shiity 10" leading/trailing shoe drums.
If you have JB5, you had a 1 1/8 bore master with the 40mm QTU.
JB6 was the same master and booster, same front brakes as JB5, but with 11.x" Duo-Servo drums in the rear instead of the shiity 10" leading/trailing shoe drums.