Brake Accumulator Runs Every 5 Mins

Not great but not critical yet. On gen3 every 15-30 seconds is bad. I would probably get a dealer to do a complete brake system bleed using Techstream scanner.

 

I believe ours does the same. I wouldn't worry about it.

 

Depends on how much failure you think is ok. I'm familiar with Gen 3, where in a system in good condition, the pressure holds up for some hours if the brake isn't being used. So 5½ minutes with nobody touching the brake is already leaking off much faster than a good system. On the other hand, you routinely see people wait until it's leaking down on the order of seconds, and trying to compare the badness of running every n seconds versus every m seconds, in a system that's supposed to hold up for hours.

But I need a disclaimer, I haven't owned or worked on a Gen 4. I'm familiar with Gen 3, and there are significant changes in the brake system between 3 and 4 that got discussed in this thread:

Prius 4th GEN Brake diagram | PriusChat

Like the Gen 3, Gen 4 has a booster pump assembly that is separate from the master cylinder/booster assembly (and in Gen 4 that's also separate from the ABS actuator assembly, which leads to this confusing diagram with two things labeled Brake Actuator).



The downside of waiting a long time when the pump is running extra often is that the problem usually hasn't originated in the pump. The original problem is likely to be an internal leak in the booster/actuator, where the pressure is being lost, making the pump have to run. Caught in time, only that part may have to be replaced.

But all the time the pump is being made to run extra often, its own life expectancy is being used up. By the time pressure is leaking down every several seconds instead of several hours, each day like that may compare to years of the pump's normal life (depending on your style of driving and how often you brake). So you could then end up with a worn-out pump in addition to the part with the original problem.

So even though 5½ minutes isn't awful yet, if this is like Gen 3 where it ought to be hours, I would already be thinking about repair.

On the other hand, if some Gen 4 owners say that 5½ minutes is typical for that generation when new, then that'd be a different story.

 

Well I own a Gen 4 which I bought new and it has a whopping 3500 miles on it. It will occasionally do exactly what yours does on the rare occasion that I leave it in READY mode for any length of time. I suggest you check out John Kelly’s videos from Weber University to learn as much as you can about the Gen 4 and the differences to earlier Generations so you can avoid unnecessary worry.

 

So 5½ minutes as the pumping interval when the brake isn't used is typical for a Gen 4?

Good to know. That's a definite difference from Gen 3. Is that covered in one of Prof. Kelly's videos?

 

May be normal. The Op says it runs a "a second or so every 5 and a half minutes" while in Ready. If it really is 1-2 second duration and the 5.5 second cycle is repeatable within a few seconds, it may well be a programmed algorithm. If the the cycle rate varies it could be an issue.

I would still consider a Techstream bleed/flush at 165k miles to ensure quality brake fluid if nothing else. When my brake booster system was failing, a flush remedied 15 second cycles for a day or two.

 

That would strain belief for me. In every earlier generation, it runs in response to pressure drop, just like a well-water pump, and that's the natural way to do it, since maintaining accumulator pressure is its job.

They could not simply program in a time-based cycle, unless they make the system actually leak enough fluid to lose a 1-2 second pump worth of pressure every 5.5 minutes, else the pressure would just keep rising until the relief valve let go.

But most people in other threads who are reporting rapid pump cycles do report pretty consistent periods when no one is touching the brake. Fluid at a controlled pressure passing through a leak of a given size may not be a computerized time delay, but can still make a decent analog timer.

 

Pretty much every system of both Gen’s are covered in his videos at one point or another. He goes through each one and in most of the videos takes whole systems apart showing individual components. Takes a lot of time but I believe helps protect one against the “Chicken Little” syndrome.

 

Does he have a video that is responsive to the question "how often does the pump run in Gen 4 when the brake isn't being used, relative to earlier generations"?

I've watched a bunch of his videos, but admittedly not all of them, and I haven't found that one.

 

John Kelly’s videos are very informative for those who desire to learn as much as they can about how the systems work. Are they useful in proving certain points in an argumentative discussion? I don’t know, that doesn’t seem to be there purpose, and I don’t have the time or the desire to use them for such purposes, so you are on your own for that.

 

They are unlikely to propose a replacement until it codes. But gen2 and 3 have brake booster failures initially recognized by frequent running. The startup running is normal.

 

Sometimes, in a thread where a specific question is being actively discussed, like "is there a difference in the normal period of pump running between Gen 4 and earlier Generations", a person will come along and post a comment like

In a context like that, the poster is often claiming that the source being cited contains something that resolves "unnecessary worry" about the question being discussed. (In the best of cases like that, the poster might say something like which video, and how many minutes in or something, so it isn't just a "go watch hours of stuff to find out if any of it helps you "avoid unnecessary worry" on this question or not.)

I guess it's possible somebody adds a post like that and only means "hey, no opinion on this question and I have no idea if this source addresses it, but it's otherwise an interesting source that I like". In a case like that, that might be a good way to say it.