I would be surprised if the booster style caused a difference in the master cylinder piston diameter...but I've been wrong before. I'd expect JB/JD/JN5 or 6 to have the metric equivalent to 1 1/8 bore (and 40mm for the third, Quick Take-Up chamber.) My JD7 uses a 1.25 bore, though.
You've piqued my curiosity regarding the bore of the MC, and slave cylinders in the calipers and rear cylinders, for the OE systems, with regard to how some of us here including me propose "upgrades" by swapping one for another.
The bits of information I posted in my prior post I gleaned from RockAuto's site, based upon description given in the MC's description and in the specs, e.g.,
- for ACDELCO 18M712 it says "w/ Hydraulic Booster" and 1.25" bore
- for ACDELCO 18M1492 it says "with Vacuum Booster; 11-5/32" X 2-3/4" Brake Shoe" and 40mm bore (only the quick-take-up bore is mentioned, not the "working" bore size as I'll call it)
- for CENTRIC 13066029 it says "11" Rear Drum; 6 Lug" and 1.125" bore; no mention of the booster type
- for CENTRIC 13066030 MC that says "13" drum 8-lug" w/ 1.25" bore; no mention of the booster type
Other mfrs' descriptions aren't very complete.
However, I've not found (yet... I haven't been thorough) any MC for 1500/2500 in my casual review that has other than 1.125" or 1.25" "working" bore size. I'll keep looking.
Meanwhile, many others (including myself) have spoken positively about the "NBS" MC. I've used a NBS MC for a 2001 Silverado and on RockAuto's site for that truck they show:
- ACDelco 18M1159, "for vacuum booster" with bore 1.338", i.e., 34mm; I used this MC
- ACDelco 18M1107 "for w/ Hydraulic Booster" with bore 1.4566929134", i.e., 37mm
Either of these "NBS" MCs has
markedly larger bores than those I've noted for the OBS.
This is all the time I have for this, for now.