Compared to my 1500, the 6 lug 2500 rotors were thicker, larger piston in the caliper, and different pads. Not sure of the specific RPO code, but by '90 those inferior front brakes were eliminated
The 1500 may also have the weak JB-3 front calipers and small master cylinder/booster. The extended-cab 1500s and the 2500s should have at least JB5 brakes--bigger caliper pistons, bigger master cylinder bore, bigger booster.
The RPO codes may have changed--JD3 vs. JB3, or something like that, but the functional difference between the xx3 vs. xx5 is as I described--bigger caliper piston, bigger master cylinder bore, and more-powerful vacuum booster. I forgot to mention the thicker rotor.
Photo 1. xx5 booster (left) and xx3 booster (right)
http://hbassociates.us/K1500_2020_Brake_Booster_02.jpg
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Photo 2. Rotor thickness difference between xx3 and xx5 front brakes. The diameter is the same, the way these rotors are positioned for the photo makes the thinner one seem to be smaller-diameter.
http://hbassociates.us/K1500_Front_Suspension_19.jpg
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Yes, the xx3 brakes were discontinued in the early '90s.
I converted my xx3 brakes to xx5 in front about a year ago.
Both the xx3 and xx5 brakes have low-drag front calipers, so you need the step-bore GMT400 master cylinder, NOT a GMT800 master cylinder!
Photo 3. Step-bore master cylinder. Note enormous piston at right side, supplying a large "gush" of low-pressure fluid to the low-drag calipers to push the caliper pistons up to the rotors. Then the smaller, high-pressure fluid actually applies the braking force.
http://hbassociates.us/K1500_Front_Suspension_54.jpg
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