I have JB5 brakes. From what I’ve heard I have the ****** rear drums, not sure what duo servo means.
JB5 is better than what my '88 K1500 started out with--JB3. You have thicker rotors, bigger vacuum booster, bigger caliper pistons in low-drag calipers, and you had the larger QTU master cylinder. Same ****** 10" (254mm???) leading/trailing rear brakes, though.
I converted my truck to JB5 in front, along with the master cylinder and booster. I put a 6-lug 9.5" semi-float 14-bolt axle in back, with 11" Duo-servo drums. Should be the equivalent to JB6 brakes, now.
The obvious difference between Leading/Trailing and Duo-Servo brakes is that the L/T has the adjuster just under the wheel cylinder at the top, and the shoes are anchored at the bottom. Duo-Servo has the anchor pin at the top, and the adjuster is a floating link between the shoes at the bottom.
L/T brakes have ONE shoe on each side that does almost all the stopping--which is why the front shoe on both sides is the one with most of the wear once the miles pile up. The rear shoe does most of the braking when you back up. Most folks don't back-up at highway speed, though, so the rear shoe stays pretty pristine. Duo-Servo uses the friction of the front shoe to cram the rear shoe into the drum extra-hard. Both shoes help stop the vehicle, forward or reverse. As it turns out, the Duo-Servo brakes on these trucks are larger-diameter than the L/T brakes, which also improves stopping power.
Good fluid flow at all 4 corners when bleeding.
If the rear shoes don't stop the drums, either there's no pressure due to a failed master cylinder, air in the system, massive leaks at the wheel cylinders, or the rear shoes aren't adjusted properly. Good fluid flow when bleeding eliminates the possibility of an actual blockage in the hydraulic system.
Given the leading/trailing shoe design, "not adjusted properly" is fairly likely. Yes, I know you're claiming you adjusted them so the drum would barely fit over the shoes. One brake application to align the shoes to the drum, and the adjustment may be way off again.
Air in the system is also a strong possibility.
I don't recall you mentioning that the brake pedal is low and squishy. I'm guessing it is. If you've got a high, firm pedal, I'm gonna be very surprised.
The master cylinder that I swapped on has a 1 5/16 bore I think. Sounds too big. Don’t think it’s a quick take up design, and i don’t know if I have low drag calipers. C555 and C554 from autozone are currently on the truck.
JB5 would have had low-drag calipers requiring a QTU master cylinder having a 1 1/8 bore plus a 40mm QTU piston at the rear. I don't know if the replacement calipers are low-drag, but it's very likely if they're spec-ed for a low-drag application.
40mm is larger than an inch and a half. Moves a whole lot more fluid than a 1 5/16 piston. Exactly what's needed to push the caliper pistons up to the rotor faces. Then the 1 1/8 bore supplies higher-pressure fluid than the 1 5/16 can provide, leading to stronger stopping power.
Don’t have a scan tool so I just engage the abs ~10 times then bleed.
That "should" work. At least, I'm told that it does. I'm not sure that I believe it, though.
I'm wondering if your ABS valves for the REAR brakes are
not activating because the rear brakes don't work (don't lock up) and the ABS for the REAR brakes is therefore still full of air. The ABS is working for the front brakes, but not the rear.
If your truck was in MY driveway, it'd get the correct master cylinder, properly bench-bled; and a scan-tool bleeding of the ABS. And then, I'd pull the drums and verify the adjustment AGAIN.
For the record, the AC-Delco "Professional" master cylinder you need is a Made-In-China knockoff. I was VERY disappointed. I was in a time-crunch, I used it, but if I had to do it over again, I'd look around for a USA-sourced master cylinder. (It may be that there is no such thing.)
https://www.amazon.com/gp/product/B001DRLC1K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
