When ABS was first introduced and getting lots of publicity ( i.e., " hype") for being the greatest advance in vehicle safety, I remember that there were after market/ retro-fit ABS units that could be installed on existing cars. IIFRC, there was a solenoid that interupted braking pressure installed on the lines at the master cylinder--one front, and one rear. I don't recall any computer control, so maybe it operated on every brake application. I've never seen one "in the flesh" so to speak, and can't recommend their use, but they did exist. I bet they might pop up at old car flea markets as NOS.