Please please help with ABS issues!

This is a longer story so I will try to keep it concise. I am just worried something may be relevant when I wouldn't have thought so.

I purchased a new to me 2014 Prius V5 with Advanced Technology Package in mid-November from a private party. During the test drive there were no apparent issues, but on the three hour drive home, suddenly the engine started seriously struggling when starting from a stop. Next day it started with the classic rough start, coolant smell. FFs. Took it to a guy that only does prius head gaskets, and for 1100 he fixed that, replaced the water pump, cleaned EGR stuff etc. I drove the car home from his house, it was a 1.5 hour drive with no issues.

Next day I took the car out again. Not even 10 minutes into the drive, all at once a bunch of lights came on. ABS, PCS blinking, BRAKE, exclamation with a circle, TRAC, accompanied by a loud, never ending, alarm beep. The car is not drivable like this. The brake pedal was hard and hardly usable on the way home. Nothing really happened at all to trigger this. Driving on a smooth road at about 25 mph. The guy who did the head gasket mentioned that the brake pump was running a lot and that I should expect it to go out soon. I wasn't expecting the next day lol.

I ended up getting an OBD2 scanner that does ABS, and came up with
C1252- Brake Booster Pump Motor on Time Abnormally Long-ABS/VSC/TRAC
C1253- Relay Of Pump Motor- ABS/VSC/TRAC
C1256- Low Pressure of Accumulator- ABS/ VSC/TRAC
Not sure if this is related:
B15EC- Replacement of Back Up Pack- Telematics

I ordered a master cylinder assembly part number 47050-47260, brand new, on the recommendation of the dealership, it is now installed in the car.
I purchased a used pump, part number 47070-12020, which is currently sitting in the box it came in.

We installed the master cylinder assembly as seen on various videos. Without bleeding the brakes at all, I checked the codes again and got some new ones.
C1203- Communication Circuit of ECM
C1345- Linear Solenoid Valve Offset Learning undone
C1451- Malfunction of Motor Drive Permission
B2060- System Malfunction of Antilock Brake System (ABS)

Then, we tried to bleed the brakes manually following a YouTube video that involved using a brake depressor to hold the brake to trigger the pump. I can't remember exactly what order we did this in, but the first two bleeders that were opened triggered the pump, and brake fluid came out. However, after the last one that worked, the pump has not come back on since. Not at any time. Not when the door opens, not when the brake is depressed, not when the bleeders are open. Reluctantly, I purchased a mini VCI cable and after MUCH effort, was able to download Techstream 14 on an old laptop. The car connects, I can read the trouble codes on the laptop, however, when I try to go through the brake bleeding process, it immediately fails. I get through the screen where it says all of the needs (E brake depressed, IG on but not ready, cable to the master cylinder reservoir unhooked etc), and when I press next, it does nothing and immediately says brake bleed failed.

I. AM. STUCK.
I have some thoughts though.
-The 12 volt battery is reading at 11.37 volts with the car off. Is that bad enough to possibly cause some of this?
- I have read in another thread that opening the brake bleeders when the car isn't in the right mode can cause it to throw codes that "lock out" the pump. Is that why I'm not hearing it at all anymore? Even so, I thought the fix for this was to bleed the brakes via Techstream, but I can't. Did the pump actually suddenly burn out (with no change in codes) or is one or more codes blocking the brake bleed utility?
- Is there any chance that communication is only happening one way? The cord is brand new but it was a cheaper one. The language on the ad is a bit ambiguous. Are there OBD2 cables that only go one way? Could it be (computer) driver related? I was able to clear codes (temporarily) so at least SOME laptop to car communication is happening. However, I tried to clear the ECU memory and I tried to do the Linear Solenoid Valve re-calibration, and neither of which seemed to do anything. The lights did not blink on the dash the way they were supposed to to show that the process was working, but maybe those are overridden by the fact that those lights are permanently on right now for other reasons?
- I was reluctant to install the new pump immediately because I heard it working when we were trying to bleed the brakes. Is that what you would do next and just see what happens?
- I checked the ABS relay in the fuse box for continuity and it's good. I am not sure if this is the relay being referenced in code C1253.

I'm so tired of working on this car. I've had it for a month and have only been able to drive it without an issue one time. I live in WI and it's a horrible time of year to be spending excess time in the garage. I would love to know what YOU would do next if this was you. I am not anything close to a mechanic, but I can follow instructions and I am tenacious. I am also broke after this absolute nightmare. I was shopping for this car for months and got completely screwed over because I purchased it at full value and then all of this happened. Please be nice.

 

Here we are, about 2 months after I purchased the car, and I finally have an update.
Based on advice about this from multiple sources, it seems most people didn't think it was the pump. So I didn't end up replacing that on my own, but spoiler, I should have.
I took it to an acquaintance who had a high end scan tool and had them try to bleed the brakes etc. They had the same problem. They even bought an official 2 day license for techstream (or the program that came after) and still couldn't get it to work. $80. He threw in the towel. My dad helped me drive it 1.5 hours on a flatbed to this guy, but at least the car was closer to civilization now. I had it towed to a Toyota dealership in a neighboring town.

My experience with Toyota was terrible. First I told them many times that the master cylinder inside was brand new from Toyota. Showed them the paperwork. Still, when they came back days later with my quote to fix the problem, they included a new master cylinder in the quote. They tried to tell me it cost $1700. They also tried to tell me the pump cost $1300. They told me the part number for the pump I had was not correct even though I took it off their TSB. The part number they were claiming it was did not even exist on Toyota's website. When I asked them to explain why, the tech advisor did not have an answer and instead sent me a screenshot of the part number on a third party website...for $700 new. The part number did not match the format of the other part numbers I had been seeing, so I'm not sure if it was an Internal part number for the same part or what. In any case, any trust I had that they were not only trying to make as much money off me as possible was eroded away by this point, so I told them thanks for nothing and paid them $200 for their "diagnostics" lol, and $120 to have the car towed to Auto Guys in West Allis.
I explained the problem to John on the phone, and while he was also skeptical about replacing the pump being the solution, he did agree that the part I had should work. So while it's not what they would normally do, because of the nightmare of all of this that I'd been through, and because I already had the part, they went ahead and tried that first. Lo and behold, it worked. After replacing the pump they were able to bleed the brakes as normal, clear the codes, and the alarm turned off. I guess in hindsight the code for the pump relay should have been a giveaway, but there is an ABS relay in the fuse box and I thought it was just referring to improper communication with it for whatever reason. That relay was testing good. But there must be another relay on the pump itself. The pump WAS running intermittently, which is why I really thought nothing was wrong with it. But when I asked my dad if a bad relay can ever work intermittently and he said yes, I started to question. However by that time the car was already miles away from me and I wasn't confident enough in this discovery to tow it all the way home and still run into trouble.
So, moral of the story, and maybe this is obvious for some; if you're having issues bleeding the brakes and recalibrating through tech stream and you have not swapped both parts, try that first before carting your car all over the world to get fixed.
I was worried it may have had something to do with the ATP- it didn't.
I was worried the OBD2 port wasn't communicating properly- it was.
Auto guys told me it's pretty important to get the correct part via VIN for the master cylinder, but the pump/accumulator are largely interchangeable between model years, which is also reflected on the TSB.

I learned so much about my car through all of this. I wish I didn't spend so much but, hey, I picked up my car yesterday from Auto Guys and drove it 2 hours home without an issue for less than a quarter of what Toyota was trying to charge me.

If my used pump fails, now I hopefully know what I'm doing next time. Hope this helps someone else in my position!