Since you want to diagnose and you said you arent familiar with pv, here's the logic to think about. Brake fluid flows from master cylinder through one side of proportioning valve, theres little valve and spring, it will always be open until X amount of pressure is built, then the valve starts to close until fully closed when panic braking. This is a generalization of the concept I havent taken that specific valve apart.
Now when they fail, if that valve isnt closing you can lock up the brakes with medium braking.
1. How exactly light or hard are you pushing the pedal when it locks up?
2. If you lift the axle off the ground can you spin the back wheel freely that drags?
3. The concept also assumes that the shoes arent touching the drums but the calipers are just touching the discs to work correctly.
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That's also the concept that I believed to be true based on a little research.
The backstory is that without messing with the brakes at all my rears started to lock up on wet pavement under totally normal braking. Within a day they locked up under nearly ANY braking, even on dry pavement. Then I changed a bunch of things that needed changing anyways, bled the heck out of it, got new grabbier front pads, and here I am. (rear shoes are new and adjusted properly, had to get new ones cause my axle seals leaked and ruined the old shoes. Got scottseals and new shoes)
1. Currently on dry pavement I have to brake very firmly to get the rear wheels to feel/hear the rears braking too hard. I can feel the initial slowdown from them then hear the tires start to squeak a bit which means they are way too tight for the wet stuff. And it rains a LOT out here. (I haven't taken them out in the wet but I would guess one or both rears would lock up at an medium-speed braking situation. So like a stop sign at the bottom of a short steep hill or when someone suddenly decides to cut you off doing 10mph slower than you are, etc. )
2. The wheel that drags changes so it's not related to one side. I also have a Detroit locker in the rear axle which makes brake diagnosis a PITA, as it turns out. I take the wheels off now and adjust the shoes until I can't slide the drum back on, then loosen them up a tad so I can slide them on. (I've tried loosening them up more and more to add play and see if that helps but it doesn't really. There's just simply too much pressure in the line or something else I haven't figured out.)
3. Discs work great now and drums also slide right on and off with the parking brake off. Also the parking brake works perfectly and is strong, another indication that the drums are properly setup.