Cars have them, (at least in the 70s and 80s
) and they were real simple. A block next to the master cylinder with the ouput of the master cylinder going in one end and a line for each axle out the bottom. Some had a rubber covered mechanical plunger was on top to reset. I spring loaded pin was held in place by equal pressure. If one or the other axles lost pressure, the pin would slam shut on that opening allowing the other to remain pressurized.
If you weren't careful when bleeding brakes, you would trip it and would have to reset it.