Brake accumulator, and brake bleed. Has an Interesting result.

I suspect you're not seeing any "air" coming out that hasn't been there since day 1. The accumulator works by pumping the fluid into a canister filled with highly pressurized nitrogen gas. You know how the accumulator assembly looks like it has two big round cylinders? One is the motor for the pump, and the other is the nitrogen canister.

Inside there is a beefy, beautiful, unbreachable bellows to keep all the nitrogen on one side and all the brake fluid on the other side. I don't know whether Mexico paid for it. But anyway, after some number of miles and years and thousands of flexing cycles, well, you can start losing some of the nitrogen into the fluid circuits.

Eventually, with less and less volume of nitrogen inside the can, the pump has to run more and more often for shorter and shorter cycles, trying to keep the pressure up. (If you have a well pump at home and your pressure tank is losing its air charge, the same thing happens.)

-Chap

 

Abs is leaking and Losing vac which is why the pump runs continuously.

 

To recap #2 a bit, the Prius brakes do not use a vacuum system. They use a pressurized fluid system, with the fluid pumped into an accumulator precharged with nitrogen gas. The bellows inside there can fail eventually, allowing the high-pressure nitrogen gas into the hydraulic circuits.

-Chap

 

The diagrams of the system in the New Car Features manual on techinfo.toyota.com are exactly what you're looking for to understand the system.

Once you have a techinfo login, all the other repair manual information is there too for your disassembly/replacement steps.

-Chap

 

Hello, my 2008 with 152,000 miles is doing this. What part(s) did you end up replacing?

 

Thank you for making this video. I am getting the pump running continuously, so I suspect I'm not too far from having to replace the accumulator. But appreciate the time and effort you put into making this available for others to correctly identify/fix issues!

 

I have brake issue on my 07 Prius. I also have the "noise" that brake systems makes..
I am confused on the terminology reference. Clearly a TSB was and has expired on the ABS Actuator
which look like the accumulator.

 

OP did you replace the accumulator.. .
...

 

FYI
Online Toyota Olathe ABS actuator pricing new at $1134. plus s/h/t and remanufacture Dorman $700

I am considering doing the above replacement . However have concerns over reinstall and bleeding complete brake system .

 

If you're talking about a second-generation Prius, whether a 2007 or a 2004, you don't have to worry about whether the "actuator" looks like the "accumulator" because in the second generation they were combined into a single assembly.

In Gen 1, the actuator is separate from the accumulator. In Gen 3, the actuator is separate from the accumulator again. But in Gen 2, it's one big box with all the pipes connected and the pump and accumulator included.

 

Yeah Gen2 Prius 2007..Let me see if I have this correct the local dealership has part for $1700 thereabout. Once replaced does that stop the noise ? I am guessing labor cost added I go to $3000 or more and three hours for pro mech and 2-3 days non pro. Would using DOT 4 brake fluid be okay while you have the whole thing torn down?

 

Here's Repair Manual excerpts. If anything else comes to mind let me know.

 

First of all hi I see so much good info on this forum its like a gold treasure. Sorry in advance English is not my first language.
I did read a lot about the brake pump accumulator issues and I start to wonder if the main problem is not the accumulator it self is failing the bladder inside ? It says for 2005 to 2009 I believe its part NOK MZ21-135 from NOK company anybody try to get one of those and just swap them out ? I will soon get a used one to study it closer maybe somebody know if it is a piston accumulator or a bladder accumulator ?
Also would it not possible to move the accumulator away like put a fitting and line and move it to a different spot in case this accumulator is not for sale but maybe a different one with same specs but little different dimensions therefore not fitting at this spot ? I hope you guys have a minute to respond I do not have this problem at this point but I am sure it will come up one day and by than I like to have a good solution for it.

 

By the way I believe this is a piston style Accumulator so if somebody has more info on this it would be great I know they getting remanufactured by DORMAN part # 587-765 at summit they sell for 650 dollars ish so there must be a source and way to service the cores.

 

I believe it is a bellows style, somewhat like this photo (though this is not exactly one from a Prius).



Toyota's report to NHTSA about the ones that were prone to failure in 2010s contained some descriptive details about the bellows.

The Gen 2 actuator assembly is a big omnibus deal, containing the pump and accumulator and also all the ABS actuating valves and sensors. When one isn't working, sometimes the problem is the accumulator, sometimes it isn't. For example, a valve that doesn't completely seal can prevent the accumulator pressure from holding.

Dorman must have the facilities to test and restore the whole thing. More power to 'em. Sounded like a nightmare, in one description by somebody here who got curious and opened one.

 

Interesting ok so piston style I will try to go after this a little I will get a broken one soon from my friends shop lets see maybe we can figure something out if there is bubbles when bleeding brakes it is a clear indicator for nitrogen leaking into to the system. Thanks for your input

 

So has anyone made any progress on this? I would like to locate a supplier for the accumulator.