Bigger bore, especially for the rears, than the stepped bore 1500 design which blows donkey balls.
IF (and I am not conceding this point, just hypothesizing) there's a problem with the step-bore master cylinder system, it's not the step-bore master--it's the low-drag calipers that make the step-bore master a requirement.
The bigger problem with the 1500 brakes is the hateful leading-trailing shoe design that doesn't self-adjust worth a crap, leading to a low, but firm pedal. And yes, rear brakes matter.
Upgrade to the bigger single bore HD2500/3500 GMT400 MC for a bolt in solution that would give the same results.
The low-drag calipers need a big shot of fluid
before they even begin to slow the car. That's the reason for the step-bore--it provides that large volume of fluid (at lower pressure) to push the pistons up to the rotor. Then the high-pressure section of the master takes over to squeeze the pads.
Many 1500s got the 1" step-bore (the larger section was perhaps 32mm.) The extended-cabs got a larger, 1 1/8 step-bore, where the larger section was 40 mm. But the caliper pistons were also larger, and the brake booster was larger and more-powerful.
Nice thing about the bigger bore is that it reduces pedal travel
A larger master cylinder bore does reduce pedal travel IF the rest of the hydraulic system is kept the same. But it also increases pedal effort required. Slapping a larger bore master cylinder onto the wimpy power booster of the "regular cab" models may not be the best idea; especially if the driver is on the smaller, weaker side of typical. OTOH, if the driver has good legs, it might be a perfectly-fine upgrade.
When it was me, I bought the parts from a Treasure Yard to put the larger-piston calipers, the larger-bore (but still a step-bore) master cylinder, and the larger brake booster onto my '88 K1500. I've essentially gone from JD3 front brakes to JD5. The conversion got stalled over the winter, but I'm expecting to get back to it in the next month or so. I'd already gotten rid of the crappy leading-trailing shoe brakes via a 14-bolt, light-duty 3/4 ton rear axle, so the rears are actually more powerful than the 1500-series JD5 brakes.
and it's my own personal suspicion that the crappy ass 1500 brakes are due to the pedal overcentering which stalls forward travel of the MC pushrod.
Gonna have to look into that. Hadn't occurred to me. My initial reaction is that GM wouldn't be that stupid...but I've been wrong before.
