Prius Gen 2 - ABS, BRAKE, and Brake System Warning light

My daughter's 2005 Prius recently developed an ABS issue. If anyone can provide any insight, I would be grateful. Details below.

Car: 2005 Prius
Miles: 77k

Problem: Recently the above lights showed on the dashboard. There has been NO recent brake work, no recent accidents, no known reasons that I know of would have introduced air into the system.

Dashboard lights(Note - VSC and "Slip Indicator Lights" are NOT lit.):
1 - BRAKE
2 - Brake System Warning light (Circle with parenthesis and exclamation point)
3 - ABS

Note: If I clear the ABS code, it comes back only "when" I press the brake pedal the first time after clearing the code, but to be clear, not until I press the brake pedal.



Codes (as read by "Kingbollen Ediag Elite"):
C1344 Hydraulic System(RL)



Indications:
1 - The brake reservoir is slightly over full. With the reservoir slight overfull, I wondered if some issue is pulling air into the system.
2 - With the reservoir slightly over full, it suggests there is no brake fluid leak… unless the system is sucking more air in than is leaking… but I have seen no leak indications.
3 - The reservoir level has come down slightly when I unsuccessfully tried to bleed the brakes, now it is maybe only 1/4" above full.
4 - When bleeding the rear brakes, a small amount would drain out the rear brakes, but I could not get the pump to actuate. I was using a "Kingbollen Ediag Elite" OBD2 Bluetooth scanner, which has ABS bleed capability, but maybe I was not using it correctly, or maybe this issue prevents the pump from operating.

Other things checked:
1 - both rear wheels removed, and brakes inspected, no visible leaks, brake pads appear normal (suggesting no pads are dragging), everything looks perfectly fine.


Diagnosis with "Kingbollen Ediag Elite" OBD2 Bluetooth scanner:
--Wheel cylinder voltage diagrams

1 - Diagram 1 - At rest, brake pedal NOT pressed, both left and right rear wheel cylinder voltages look like this at 0.43 volts:


2 - Diagram 2 - With brake pedal pressed, the other 3 wheel cylinder voltages respond. Take note that both front brakes show about 2.8 volts. The RR(Right Rear) cylinder voltage rises to 1.3 volts, BUT… the RL (Rear Left) brake cylinder volts stays low at 0.43 volts. See orange arrow.








2 - Diagram 2 - With brake pedal pressed, the other 3 wheel cylinder voltages respond. Take note that both front brakes show about 2.8 volts. The RR(Right Rear) cylinder voltage rises to 1.3 volts, BUT… the RL (Rear Left) brake cylinder volts stays low at 0.43 volts. See orange arrow.



3 - Diagram 3 - Shows RR wheel cylinder voltage shows good fluctuations with brake pedal pressure when brakes are pumped several times:



4 - Diagram 4 - Shows RL wheel cylinder voltage DOES NOT fluctuate when brakes pressed, but merely shows a small "blip" in voltage, and stays low at 0.43 volts:



I had posted earlier on another post named "Help Diagnosing Persistent BRAKE, ABS, VCS, and ((!)) Lights", and Tombukt2 had said " This is all actuator control and that lil pump looking thing on pass side firewall . I believe is rear brake pump or control.". I think Tombukt2 may be referring to the "Stroke Simulator". Thank you Tombukt2, I was not even much aware of that until you mentioned it.

And Brian1954 said I should create a new thread, so that is why I have created this new, and more detailed, thread. Thank you Brian1954.

Meanwhile I am trying to learn more about the system so I can narrow things down, and prove which component failed, before I start replacing things.

Thank you all for your help, and patience reading this long post.
Mdixon1