I completely agree about mud/water and older vehicles with 4 corner drums, both can be scary, been there done that. I've daily driven and wheeled a good chunk of vehicles with manual 4 corner drums. My daily for a long time was a 56 Studebaker truck, manual drums, Armstrong steering, etc. And my first wheeling rig was a M38A1 that I put a powerful V8 and overdrive in, 100MPH, 81" wheelbase, 36" tires, and 4" of lift don't go well with front drum brakes.
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Disc air brakes have been an option on heavy truck for a long time but very few use disc brakes. For them it's a money calculation more then anything else, while disc do stop better and are less hassle they also cost more initially and more important are actually heavier then drums. For OTR truckers weight is HUGE, every pound adds up, it's not uncommon for the weight of fuel to put you over. Also like us discs have a parking/emergency brake issue, for us it's manageable, for a 80,000lb semi less so. Discs do stop better, last longer, and fail inspections less though, so they are become more popular on rigs that see a lot of stop and go, where a few hundred pounds matters less, and where it's someone else's money, like government utility and refuse trucks.