brake valve fail open conditions/consequences

Alright I've got a 2005 with about 150,000 miles on it, a copy of techstream, and I'm stranded in Manhattan New York about 150 miles from where I live. If you have a prius guy out here please let me know.

DTA CODES
C1252 Hydro-booster Pump Motor
C1256 Accumulator Low Pressure

I think C1256 is irrelevant, all the pressure is just discharged and can't be refilled. For C1252 the repair manual diagnostic flow chart is linked below.

My prius's braking system is in "failsafe" mode where the isolation valves are failed open (valves between the stroke simulator and FL caliper, master cylinder and FR caliper), I only have front brakes, the beep is sounding, the ((!)) ABS VSC lights are on. I'm not sure if regen braking is truly off, haven't tested as I don't want to drive.

If anybody has a complete knowledge of causes and consequences of this failsafe mode PLEASE share. I'm wondering if it engaged because of a bad motor or if something else caused it to engage and now the motor is prevented from functioning. Especially because the brake system has a recent history and recent repair:

Full story: my rear brakes were grabbing in the rain, I opened them up to put silicone grease on the contact points, it turns out the friction material on my rear right shoe got stuck and upon taking off the rotor it all fell off. I drove the car like this for a while maximizing regen braking, letting the shoe just grind on the rotor otherwise, and it was fine. Later I have my first encounter with "failsafe" mode where the brakes were hard, its beeping, etc, and I cleared it with the 8 pump paperclip jump trick and had no problem. I figured the system was overreacting to something that WAS weird with that rear passenger drum brake. A month passes and last Wednesday this happens again but the paperclip clear doesn't work, so I resolve to carefully drive to Autozone for new shoes and a new rotor, as I drive it clears. Now the mode is back, I've changed the brake shoes, and the C1252 cannot be cleared.

I've tested the pump relays MotorRelay1 and MotorRelay2 and they're fine. I wish I could directly stimulate the motor with techstream but I don't think I can. I wonder if bleeding the brakes or initializing the linear solenoids could possibly help. Perhaps I damaged the cylinder while working back there?

The pump showed no signs of dying, it wasn't running for abnormally long or too often. My suspicion is that the recent history with the brakes caused confusion in the system somewhere along the line. I want to narrow things down completely before I cave and replace the brake actuator module.

Knowing exactly the consequences of failsafe mode (does it disengage the pump motor?) and causes would be very helpful.

Thanks

Btw my 12V battery is 12.4 measured while off in the back, not sure if that's low. Tried to measure it in the front in the fuse box but it seemed to be oscillating, if you know whats up with that let me know.

Links: nevermind i cant add links as a new poster!
DTA C1256
paperclip clear
helpful video on brake system
There were many helpful forum posts on this topic but none seem to address my exact issue beyond this point.

 

Updates:
First off I meant "DTC" not "DTA".

Anyway I realized I was only scanning the ECU for the ABS/VSC/TRAC. I did a full "health scan" and it revealed that in the Transmission Control ECU there were DTC codes C2300 (Actuator System Malfunction) and C2318 (Low Voltage Error (Power Supply Malfunction)). For what it's worth there is a B1421 in the Air Conditioner ECU.

I cleared them, rescanned and got just my two ABS codes back. But it makes me wonder about the 12volt battery triggering something strange.

I also noticed some weird behavior from one of my ABS motor relays. It would be on when it shouldn't've been and when I'd test it it'd continue to behave sort of haphazardly. I swapped it around with the other ABS motor relay and it continued to act that way in the new location. Possible this is related to low 12v battery? I took it out to test it and it functioned fine.

I didn't write these numbers down but I checked my 12v voltages using the headlight flip diagnostic menu and I believe it was 12.4 in ACC, 12.0 in IG-ON, and 14.4 in IG-READY

 

I did this, it's pretty explicit in the OP. Blink codes are redundant given the DTCs but for anyone referencing who doesn't have DTC scanner access, they were ((!)) 57, ABS 42, VSC 45.

 

Thank you this is a great point. I did screenshot the freeze frame data and it is telling. We're talking ~6.4v in the C2300, ~8.3v in the C2318. I looked at the current data after and it was a normal ~14.2v. Pics attached for these datapoints.

I don't understand the 12v electrical system well at all (eg DC/DC converter and all that) and I'm not sure why it would've dipped like that.

 

What worries me is that it's brought up often enough as a possibility, but I don't recall seeing many success stories for this brake system issue as a result of replacing the 12v battery. Perhaps it just wasn't so strongly on my radar when I was browsing days ago though.

I think my order of operations after I get the car towed home will be replacing the 12volt battery then replacing that bizarre ABS motor relay (I wonder if I damaged it while testing it, holding the oversized jumpers and the relay all myself was a balancing act that led to the relay turning on and off extremely rapidly; the circuit was closed but still floating in the air -> a brief very rapid series of oscillations that maybe it wasn't made for). I can travel down the C1252 diagnoses (I should be able to link these tomorrow once I'm less of a new member. It's a lot of circuitry connectivity stuff).

Anyway, from there maybe initialize the linear solenoids? Maybe bleed the brakes / make sure I didn't damage the rear brake cylinder? Work my way up the line until I may need to replace the whole actuator assembly. Suggestions welcome.

 

I just found that failsafe brake mode does not inherently mean that regenerative braking won't function. I'd seen the opposite claimed.

I wonder if the mode directly prevents the motor that charges the accumulator from running.

And thanks ChapmanF. Havent tried anything yet this week.