J55 HD Brake size help
 

I need to purchase new wheel cylinder, however, I am unsure as to the size. Most places have the options of 15/16 or 1 inch bore. How would I go about checking that? Or find out from the J55 HD BRAKE SYS code on my build sheet?

Re: J55 HD Brake size help
 

Call parts store they should ask you the rear drum size. Use the listing below to get the correct size. That's what I did. All good now. I have the JB5 on my truck.

According to the Chevrolet 10-30 series light truck service manual:

JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm

JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm

JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost

JB7 is basically Dana 60 Front disc with the smaller 14FF drum brakes with a vacuum booster and JB8 is Dana 60 front disc with the Large 14FF drum brakes with Hydro-boost.