ABS Sensor or controller issue?

Have you checked fault codes? With scanner capable of talking to all control units (not just engine)?

Check the signals from wheel speed sensors with a scan tool when car is driving straight (at pretty slow speed) and (slowly) slowing down. That tells you if the problem is related to a abs speed sensor or exciter ring for abs speed sensor. Then you would do some other checks to confirm that it's the sensor or ring and not something like wiring (or very unlikely the controller).

Speed sensor is easy to replace if you can get the bolt out.

For the ring the problem would be most likely a crack in it causing a one cap to be picker than others. I have seen that on other cars. In a Prius and in the cars I have seen this it's a part of front outer cv-joint. Ways you can check it are ether just visually checking it (which in a Prius is kind of hard with no disassembly as there's a shield on top of it). Or using a scan tool and/or oscilloscope to check it. You might be able to get a ring from somewhere and then you could just replace that and leave the old CV-join (and axle) in a place.

 

Below 6 mph the car is using hydraulic service brakes. Above that threshold, the car is using a combination of the hydraulic brakes and the electric regenerative brake system- the computer chooses based on a few different criteria.

Anyway, the electric brakes simply don't work below 6mph, so the traditional hydraulic brakes have that duty, and something is apparently wrong with them. This could be a minor mechanical problem like deposits on the rotor face, or a deeper problem with the hydraulic controller.

You might try cleaning/re-bedding the brakes- the idea is to do a sudden hard application of brakes from highway speed to use the pads to scrub off any rust or pad deposits that have collected on the rotors. It only costs a few minutes of your time, so it can be a useful first step.

Do this on a quiet stretch of road where you won't impact traffic. The idea is to practice a panic slowdown (not stop) from say 50-10mph. When you stomp the pedal fast, the computer will recognize this as a panic stop and use the hydraulic brakes in place of the regen system. Don't come to a complete stop; this could actually leave fresh deposits on the rotors.

If you aren't comfortable doing the re-bedding procedure, it's time to put it on a lift and do a physical inspection of the front brakes.

 

Leadfoot J. McCoalroller has a good point. But you should also feel the effect at slightly higher speeds if that's the problem. Especially with harder braking that uses more friction brakes.

 

I wouldn't just replace the sensor without first looking into it. Just checking the signal with Techstream software or good scan tool is not that hard.

But if you're just going to replace the sensor spin the axle around and look for the damage in the ring. You can see the ring from the inside or from the hole where the sensor was when the sensor is removed.