Prius brake issues

Prius brake issue

I’ll just cut straight to the chase. I have a 2010 Prius, ~212k miles. Runs great. For the past month or so, there has been this hissing noise from the brake pedal whenever the brakes are depressed continuously, say for sitting at a stop light. Not a long single hiss, but back to back smaller hisses (hiss, hiss, hiss, hiss.) Been meaning to get it checked out, but my mechanic has a consistent week+ backlog, and I’ve been busy (aren’t we all.) Regenerative and mechanical brakes have been functioning totally normally otherwise, no warning lights. So, cut to yesterday. Out of nowhere car starts exhibiting a metal on metal grinding noise whenever the car is moving. Noise is emanating from the passenger front tire. I pull off the wheel, and the pads are very low. So, I change both sets of front pads, noise disappears, problem solved.

WRONG.

Pull out of a parking spot this morning, and suddenly my Brake, TCS, and ABS warning lights are illuminated, and car is no longer doing much, if any, regenerative braking. Car will still stop, but you have to depress brake pedal much more firmly to engage the mechanical brakes near the end of the pedal travel. Doesn’t feel unsafe, per se, as the car still brakes strongly, but it just feels like a normal car, no hybrid trickery anymore. Also, the hiss is totally absent now when sitting still with the brake pedal depressed. Mechanic is talking about accumulators or some such, and throwing around numbers like $3000 to repair. For the love of god, let there be some Prius expert on here who can maybe point to it being something even marginally less expensive. Also, from what I’ve described, is the car unsafe to continue driving as long as the mechanical braking system seems to be functioning just fine? I know this has been a smart novel to read, so many thanks in advance for your time and expertise.

Edit: Just had codes read. C1391

 

No and, to his credit, he said it could absolutely be something simple. Said he wouldn't know until he had it in shop. But, going off my description, that's just what he said his gut was telling him. I do trust the guys though, it's a very reputable shop in Raleigh that does a ton of Prius work, and their reputation with customers is pretty stellar. They've always shot straight with me, did some excellent work on my gf's car as well. I don't know if the brake ECU codes are different than the more generalized C1391 that I pulled at Autozone.

 

So, got the car back today from mechanic, they diagnosed it as originally surmised, bad accumulator and actuator assembly.

To answer your questions, I did do the brake job myself, and I didn't open the bleed valves. The electric hum of the pump probably lasts for 5-10 seconds and then cycles every 20-30 seconds any time the car is on.

Brought the car home today, as I just don't have 3k to spend on it. Cleared the codes with a Bluetooth OBD reader today, and hiss returned. Warning lights returned within 40 miles. Going to attempt the repair myself. Got an OBD II dongle and techstream coming tomorrow. How do you locate the part number on the actuator assembly without disassembling? Can't see much in there, even with a flashlight.