I have a 88 Chevy k2500 with a 5.7. I need to replace the brake master cylinder but the parts identification sticker is rubbed out of the glove box. Any help to figure out which brake code i have? This truck is the 6 lug 2500 as well (heavy half ton)

 

My best suggestion is to measure the rear brake shoe size. 10 x 2 1/4 JB5/JD5 11 x 2 3/4 JB6/JD6 13 x 2 1/2 JB7/JD7