I worked as a brake development engineer on these trucks, so I may be able to sort this out for you as well.
I'm not finding those numbers, either.
A little more detail would be helpful, like model (ie, C10, C20, C30, K10, K20 or K30) and GVW if you have it. Gas engine? Diesel?
https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1986-Chevrolet-Truck.pdf
The actual code you refer to is "J55" which is the "merchandised RPO" for the "Brake - Heavy Duty" system (ie, the customer would choose this). The "non-merchandised RPO" would be JB5, or JB6, JB7, or JB8, and is chosen by the engineering release charts.
Those are the codes we are trying to get at.
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