Check PCS System, and multiple lights on

Hello everyone. I have a 2012 Prius with 134,000 miles on it. I am now getting a Check PCS System on the screen, with multiple lights on: PCS, Brake, ABS, little exclamation mark in a circle, car with skid marks and the exclamation mark in the triangle. I haven't noticed any changes in how the car is actually breaking. We had this first happen right before our last oil change but we added more brake fluid (which has remained full), the mechanic reset the car, my husband turned the ABS off with the button below the steering wheel, and we didn't have any issues. I am up for another oil change after 3,500 miles and the lights came on again last night. I have read a former post about someone who fixed this problem by replacing the rear right wheel hub assembly as it contains the sensor. Just looking for general guidance. I am not able to fix the car on my own but I found that with my last problem, you guys had great advice and can often lead me to guiding my mechanic to get to the root cause of the problem much faster! Thanks in advance!

 

So... today I got my oil changed and had the mechanic take a look. Funny enough, last night ALL of the warning lights on the dashboard decided to turn themselves off after I used the car for a short drive and let it idle for 5 min! Today, the mechanic said that there were no error codes that came up. He did however find a string wrapped around something under the car and maybe that set the sensor off. He took the string off and also cleaned the sensors in the front (said there was some sand in those). He said he took a look at the rear wheel sensor but didn't see anything off, and without an error code, he didn't want to fool with that. Looks like if it happens again, we'll need to take it in quickly while the lights are on.

 

I just had the car turned on but I wasn't moving/driving.

 

So, the lights came back on when I turned my car on this morning. I took it to my mechanic so he could read the error code. He got C1391, and said that this most likely meant that it needed a new anti-lock brake system. He isn't equipped to do this, so he didn't have anywhere to suggest accept the Toyota dealership. Any recommendations? He also refunded me the $80 that his employee charged me for diagnosis the other day. : ) I also noticed a new sound that was there this morning - he said it was from the brake system (not constant but intermittent whoosh sound that doesn't stop).

Any suggestions? Is this a common problem with 2012 Priuses? It's an older car but only has 134,000 miles on it. Any guess at what this repair costs at a mechanic / at a Toyota Dealer? Our closest Toyota dealer is 45 min -1 hour away.

Also, I did have new brake pads put on (work done on breaks) less than 6 months ago. I wonder if this would have had an effect on the system and if I should follow up w mechanic who did that work. It was a different mechanic than the one today.