went to change the master cylinder on my K20 today. first thing i did was went to the glove box, and checked for the option codes. "J55" great, sounds like your asic RPO code... so i get to the parts store, and the girl behind the counter (very cute, and very knowledgable) looks it up. option codes it specifies are JB6, and JB7. my truck has neither. so we decide to go for the one with the larger bore, since it specifies "heavy duty brake system", just as the "J55" RPO code does. i get it home, tear the old one out, and put the plugs that were in the new one into che core. i then search the box for the bench bleed kit. not there. i've gotta retun the core anyways, so i'll just go ahead and pick one up while im there. no big deal. after i return, i bench bleed the cylinder, and install it to the booster. as i go to hook the lines up, son of a bitch, the line on the front resivior is too small. so i've got 5/16" lines on both the front and rear resiviors of the master cylinder. anybody know if the "JB6" master cylinder has 5/16" lines at both ports? BTW, the JB6 cylinder had a 1 1/8" bore, and the JB7 had an 1 1/4" bore. my stock cylinder turned out to be a 1 3/16" bore. which is better for the disc brakes that i plan on swapping to the rear eventually?