Gen 3 Known Brake Problem - outside of the ZJB program terms

As it turns out, it's good that I started looking into this when I did. I was away on business for two weeks (by plane), and upon my return when I started my Prius for the first time, the Brake, Traction, and ABS warning lights came on within a minute of starting the car. The only code was C1391 - Abnormal Leak in Accumulator. The brake pedal was softer than usual, but it continued to brake fine in normal (non ABS) conditions.

I cleared the code and the car went back to the pump cycling every ~10 seconds as it had been doing before and has been that way ever since (4 days now). When doing live monitoring on the system in Techstream, the accumulator sensor voltage fluctuated between 3.25v and 3.7 volts in sync with the pump cycling (with a slight lag, of course) -- voltage increasing as the pump ran and decreasing after it stopped running. The fluid in the reservoir rises and falls with the cycling of the pump as well.

I think I have read every thread in Prius Chat on this. Based on what I've read, I believe that I need to replace the ac-tu-a-tor/master cylinder/booster unit. I base that on the fact that the frequent ac-cu-mu-la-tor pump cycling was happening when NOT braking and based on what I read in:

this info from @ChapmanF Brake Booster Malfunction | Page 2 | PriusChat
and
this info from @Mr. F Brake Booster Malfunction | Page 2 | PriusChat

The only thing that I'm not sure about is both of your references to a stethoscope being able to identify both conditions. I've scoped both units but I don't know what I should be hearing. I don't hear any obvious sounds. I don't hear anything at all when touching the accumulator "cans" or the brake line (red) coming out of it. I do hear subtle fluid movement on the actuator in the 2nd and 3rd brake lines from the top of the unit....moreso in the 2nd line....right after the pump stops running each time. I would have expected to hear more in the 3rd line (the red one) because that one goes to the accumulator, but whatever. Nevertheless, since the only sounds I hear with the scope are at the actuator, am I pretty safe to assume that's where the problem is?

Thanks for all of your great info so far!

 

Thank you @ChapmanF. I appreciated all of your contributions throughout these threads. VERY informative.

I should also mention that my accumulator unit is not one that was on the naughty list in the '10 and some '11 models. The yellow label is in the "good" direction so that helped steer me away from that as being the culprit too.