Hi everyone. So I started noticing that my brake accumulator frequency had increased in the last couple of months. Right now, the pump runs almost like clockwork at 34sec intervals. (When car is stationary and the brakes are not being used). I have read almost every thread on PC about brake accumulators and actuators failing and recalls. I know there is a TSB out too, but the vehicle I am talking about is 2010 so it doesn’t apply on it. FYI, I replaced the actuator last August from a totaled car. What I did NOT change is the accumulator. Well, when I started reading out, realizing how the brake system works in a Prius I just thought I should try changing the brake fluid. What I did was: I flushed all the brake fluid a couple of weeks ago with brand new one and bled the brakes. Outcome: No abnormal whirring of the pump! I waited for 15 minuted while the car was in Ready mode and the pump never worked once. It worked on every 3rd/4th pedal press but as long as I didn’t use the brakes it would never pump. I was so glad that my problem was fixed. The Real Result: When a couple of days went by, it started whirring again but the interval was delayed, much longer, every 3 and a half minutes or so. After 5 days, it become more regular around a minute or half or so, and now it’s back again to around 34 seconds. What I don’t understand and need opinions is: I know there are around 12 valves in the actuator, and around 2 in accumulator and overtime these valves fail either in the actuator or the accumulator releasing pressure abnormally. Okay I get it, it’s a wear and tear thing, these valves have a life and they fail one day. BUT....if any of the valves were leaking, then how can they NOT leak with the new brake fluid? Why did the new brake fluid or the brake bleeding process “fix the leaks in the valves in the actuator or accumulator”? This makes me realize there could be no leak in the valves and in fact the bellows inside the accumulator is failing which is allowing nitrogen to mix in with the brake fluid thus acting like air in the system and my pump has too work more often. That’s why it got fixed until the nitrogen started to mix again with the fluid, and pump gradually started to work more and more. I need someone’s opinion on this so I can start sourcing out the replacement parts. But all I am trying to say is if the valves leak, they leak all the time right? Bleeding brakes shouldn’t fix them for some period. But if nitrogen mixes with the brake fluid, yes then it all makes sense.
Hate to say it, but I'm not sure that's actually possible now- not because of the nature of the brake problem, but rather because fuel prices are that low. Our 2018 will never pay for itself vs. the nonhybrid cars we were considering. We got the prius anyway because it was nicer. I've seen great brake system advice from @ChapmanF before, hopefully he'll see this notice and visit your thread. (you can also use that as a search key for some background reading) Good luck!
Update: Replaced the accumulator today. Replaced all the brake fluid. Pump now only runs if brake pedal is used and the duration is much lower when you open the driver’s side door after a while it has been parked. But hoping it stays the same. Let’s see what happens in a couple of days. One more thing, while I was bleeding I forgot to top up the reservoir and I realized it only when my friend tried to pump the brakes and they were all squishy. I quickly topped it up. When I continued bleeding, there was a lot of air trapped because it ran dry for a fraction of time. I don’t feel any problems with the pedal right now, tried activating the ABS on a vacant road and it worked fine. No error codes. But I don’t know it’s me being paranoid, I feel when the car’s rolling, like at 5 mph, I press the brake pedal up to 25% of the total travel, it feels odd like it’s not stopping fast enough like it used to. Like you had really really bad brake pads that kind of feeling. Again, I don’t know if I am being paranoid right now but overall the pedal and brakes work totally fine.
After the air was allowed in, did you do the complete bleed process intended for replacement of the actuator?
No. How do I do that? I put the car in invalid mode just like in this video () and bled all four wheels.
The brake bleed utility in Techstream has two options, a short bleed for usual cases like you had a wheel cylinder off, and a long one for when the actuator has been replaced. It will step you through doing both rears, both fronts, both fronts again a different way (there are two separate sets of hydraulic passages that have to be bled with the actuator valves set different ways), and empty and refill the accumulator about six times. It can take long enough to do that you should make sure the 12 volt battery is very well charged before you begin, and/or connect a 12 volt supply to it for support. But the idea is to end up getting the actuator cleared of air. If air gets sucked in part-way through, my inclination would be to start over.
I would go for the second option, avoids the need to rush, and who knows, you might need to do it over? A charger's good to have on hand too.
I'm not sure a 'charger' there will do much more than slightly prolong your working time: if you have a charger that you have chosen appropriately to use with a Prius, it probably has an output of 5 amps or less, and that's lower than what the car will be consuming during the brake bleed. Of course it is better than nothing and won't hurt. I would still begin with the battery as juiced up as I can arrange, anyway. At a dealership, they would hook up one of these, which is capable of fully supplying the operation of the car (at least without all the headlights, defrosters, blowers and such ... let's be reasonable), so nothing is being drawn from the battery.
@ChapmanF @Mendel Leisk thank you so much for these insights. Just wanted to give an update. A friend of mine has TechStream set up. We’ll be doing the actuator bleed next week. He was busy this weekend. I actually did another brake bleed yesterday after having it drive all day. And now the pedal seems to be working extremely good. Actually, pedal was always good I think because it’s the “actuat-or” that actuates the brake pedal feel so even if you have a little air in the system it will like a rigid pedal. But still after doing this 4th bleed in total, car stops really good while crawling now, just like it used to before all this experimentation. But I am still doing the actuator bleed just to be sure as @ChapmanF advised. All this aside, the main experiment was to see any change in accumulator running frequency. And it’s still early to say something but let’s see how it all turns out.
Guys I looked all over the internet and I am so surprised there was not a single tutorial on how to perform stroke simulator bleeding. Why is that so? People don’t do it? Or is it so hard? And most importantly how do you do it? LOL.
You have a Gen 3? The stroke simulator is attached to the actuator. Techstream will tell you when to bleed it as part of the actuator bleed sequence. Make sure the cowl is removed before you start so you can reach the bleeder.
I am having an extremely similar issue to this, would you say instead of whirring its having a hissing noise? the hissing noise like air escaping when depressing brake pedal but its much worse than it originally was before changing my calipers.
How did you go about bleeding the system after changing your calipers? If in any doubt, I would revisit that procedure before anything else.
I posted a whole thread about it, basically just sucked it out with a tube, not a joke. The brakes definitely still stop the car, I just don't know why this sound occurs.
Which calipers did you change? What was the reason for changing them? Was there any chance the reservoir got low while you sucked fluid out with the tube? How do the brakes feel if you do something to trip the system into fail-safe mode (pulling some ABS fuses ought to do it)? You can get a false sense of security from how the brakes feel when the actuator is in control, because it will always achieve target pressure even if there's air in the lines (though you might notice something like a hissing sound). What you don't want is for the brakes to drop into fail-safe mode for some reason when you're not expecting it, and have that be the moment you learn whether there's too much air in the lines to stop the car with straight hydraulics.
I need to change out my brake boost/master cyl assy as well as the booster pump. I simply don't have the funds to buy techstream and the cable so I was hoping to do it manually if possible? I assume I would do the long bleed you noted above, any chance you would be able to provide manual instructions to do that task? Probably will take quite a bit of Dot3 but its cheaper than any other options. Thanks
You could try "Invalid Mode" but I would then get a shop that has a proper scanner to run the procedure the right way. Invalid Mode: Press parking brake Power Button twice Press brake pedal and go into Neutral Press brake x8 in 5 secs Press Park Press brake x8 in 5 secs and release Press brake pedal and go into Neutral Press brake x8 in 5 secs Press Park and note Blinking brake light You are in invalid mode Bleed rear brakes by holding pedal down Bleed front brakes by pumping brakes Power down to exit invalid mode Consider using charger on 12v battery during procedure
I'm not aware of any reason to think there even is an equivalent 'manual' way to do that task. The air needs out of all the internal passages, including ones behind internal valves that are controlled by the Skid ECU in a procedure commanded by Techstream. The 'invalid mode' procedure suggested by rj might get you braking enough to get the car to a shop that has Techstream and can do the proper job. It might be a good idea to make that trip to the shop during off-peak traffic times or on less-traveled back roads. If you can scrounge an old Windows laptop somewhere, doing this yourself using Techstream might not cost you any more than you'd pay a shop to have it properly done.
