Prius V (Alpha) faulty bleeding or bad actuator?

Hi!
First of all, sorry for my bad english. I'll do my best
I've read almost all of threads about c1345/c1368 codes, bad actuators, but i didn't see same story like mine.
I bought a Prius V/Alpha (called Prius "Plus" in Europe) to my wife. It was a surprise.
I'd checked the car for hours, everything seemed to be okay. There was 2 DTC when I checked it with Teschstream: C1214 (curr. conf.) and B1585 (history). I tought there was a faulty servicing method maybe or whatever, so i didn't really care about these codes. Moreover, occassionally there was a little hissing sound around the brake master cylinder, but it didn't make any sense for me. However the seller itself said: "wow, what is this hissing sound?". There wasn't any warning lights on dashboard, no MIL (check engine), everything seemed to be OK. We went for a testdrive aswell, the seller was the driver. He asked me to try it, but i refused (this is very important part in the story). Why should i drive? I' ve driven lot of HSDs before, i will hear everything from the passenger side too (suspension knocks, engine rattling, etc.). This was quite bad decision, think by now.
So i bought the car. I had to wait almost a month because of paperworks, new license plate, registring the car, etc. Our country is a little complicated
However, i'd finally got the keys, about two weeks before now.
Before start my journey to home, i checked engine oil level, etc. When i did this, i heard the hissing noise from the actuator again, but i still didn't get excited about that. Jumped in, READY, go.
While was driving the car, the hissing come more frequently. Maybe low on A/C gas? That could cause that sound, i heard that before from someone.
And no. I'd figured out if i DON'T press the brake pedal, the hissing is ON. When i press the brake pedal, hissing is OFF. Moreover, when i stopped at traffic lamps, and begun to disengage the brakes, the pedal felt "kicking back"/"pulsing" at my feet. Same feeling at light braking on slopes. Wow.
None of calipers/rotors was hot, there wasn't any warning lights on dash. Apart from kicking back pedal feel and hissing sound, braking felt normal.
Meanwhile i called the seller. He said his "mechanic" had been changed brake fluid, but he doesn't really think it was made with Techstream. The brake fluid change made before our first meet, so the hissing sound could be caused by the mechanic's incorrect bleeding procedure (? could be? are there any chance at all?).

So, when i came home i did the bleeding with techstream. Bleeding was succesful via Techstream, but hissle sound stayed with me. And the accumulator still works much more than usual (because of the accumutator's internal leak[?] or stucked air[?]. Still didn't have warning lights at all! Theoretically bleeding was done, but hissing was still there.

After of these, i tried to do "learning linear valve offset", and this is where i stucked. It does the valve offset almost for the end, but there isn't any quick and slow flashing abs/brake icons on dashboard when it "finished", only when it's proceeding. Since then C1345 and C1368 are my new friends, because of linear solenoid valve offset undone. However C1214 is gone. I tried bleed the accumutator itself too, but Techstream alway lost connect at this procedure. However i managed to get some fluid out from bleeder of stroke simulator, when TS get it for seconds. But I couldn't see any of bubbles.
I think "linear valves offset learning is undone" is caused by what is doing the hissing sound too...

The question is: are there any possibilty that highly classified "mechanic" could kill the abs actuator itself, with his unknown bleeding method? Maybe I have any chance to bleed out/flush correctly, if i do it idk...20x times the bleeding itself? Or when hissing sound coming from actuator, it's already late? There is only one solution, replace the actuator unit (or the accumulator too)?
I know the actuator is common problem, but this is too weird for me. The seller drove home the car about 1000 kilometers (~600 miles), he must recognize the problem when he drove this long way. He is absolute transparent seller btw, i really don't think he wanted lie to me. That's why i think the "mechanic" did something wrong. What do you think?
Thank you if read my short story
Car is a 2012/08 Prius Plus (7 seater), ODO: ~268.000km (~168.000 miles).