Clear, Consise Brake Upgrade Instructions, please.

[MountainManJoe]

Go ahead and speculate.

fo·rum, n.
A medium for open discussion or voicing of ideas.

It's fair game if you don't present it as fact.

[MountainManJoe]

I see... so our master cylinders have some additional reserve capacity. Good to know.

It would seem that heavy trucks still use drum brakes because they offer more stopping power than disks that would fit in the same space, due to surface area and self-actuation.
Disc brakes don't fade as badly, but drum brakes are more economical.

[Meterpig]

From working in the trucking industry, drum brakes are widely used because they are cheap and easy to repair. They overheat quickly, vent badly...but hey, you can find parts almost anywhere.

There was talk of upgrading to disc brakes a few years ago to meet new standards. I think there was also talk of upgrading to coil over front suspension.

I am pretty sure rear drums that are larger will work than smaller, but rear discs are the ultimate.