[Honest Bob]

I would verify why kind of brakes you have front and rear and try to match the master up from there. If your brakes worked great at one time with the JB7 master you could of course just use that.

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

I suspect the boosters between jb5 and jb7 might be interchangeable but I am not 100% sure.

If you can identify what rear axle you have you can assume some things.

If its a semi float 14 bolt its probably got jb5 rear brakes.

If its a full float 14 bolt then its probably got jb7 rear brakes.

A one ton 14 bolt does not have the same spring perch width as a 1/2 or 3/4 ton so you probably do not have jb8 rear brakes.