Weird noise when brake pedal is pressed at stop

Without viewing your video ... would you maybe describe the noise as a quack, honk, or bark? If so, search on any of those words with the word brake and you will find many threads.

 

I actually had the same problem. Just wondering if your brake fluid is low. I purchased $5 fluid and it seem to help.

 

This a Gen 2 thread and your Gen 3 experience might not carry over to a Gen 2 as the Gen 3 setup of this area is different.

Please be mindful of which forum you're posting in.

 

Oh yeah, I know that one. It's a barking spider* usually located on or underneath the driver seat. Put the driver side window down, air vent to high and try Beano before you eat. It worked for me. LOL

OK seriously, I had that a few times and what I found was that I cleaned and lubed the guides and shims for the brake pads and the slide bolt. I tried a variety of lubes marketed for this application (disc brake guides) but the stuff that really worked for me for more long term (kept the noise away for years rather than months) is the genuine Toyota Lithium Soap Grease, also referred to as rubber grease (for moving rubber components like sliding o rings etc) and the Toyota Disc Brake Grease (sometimes referred to as anti squeal grease).
And you will need some brake part cleaner, brushes, cleaning tub (oil change pan if it is really clean) and the usual tools for this sort of job.
You might need new boots on the guide pin too. The after market boots work OK, but don't fit quite as well.

The Toyota part numbers are:
Rubber Grease 08887-01206
Disc Grease 08887-80609

These lubes are not cheap, and even Toyota service bays often use cheaper alternatives. But that's what worked for me.
I also found this worked well for "grabby brake syndrome" at low speeds (below 5-7mph) when the mechanical brakes are in use exclusively.
NOTE: if it sounds like I am writing in a foreign language (Klingon is my native language) I can explain this in non car geek terms, just ask.

When you are applying the anti squeal grease between the shim and the pad also apply a SMALL amount to the disk pad support plate where the ears of the pads slide back and forth. It makes a big difference, but a small amount is all that is needed so it does not attract dust and dirt. And don't get ANY on the braking surface of the pads or disk.
In the 2009 service manual, Volume 2, section BR page 32 has a very useful diagram showing where to use each type of the lubes I mentioned above along with the torques needed on some critical bolts. I think the 2008 is about the same page.

IF you are used to working on cars and light trucks, it's fairly straight forward to do the lube on the shims and guides and slider pin. And that might be all you need. BUT, another place that needs lube is the seal ring on the brake piston. And that is considerably more involved but totally doable by a shade tree mechanic with patience. Getting to the seal on the push piston involves bleeding the brakes which is "a bit involved" and should be done with either Toyota Tech Stream or a similar system.

Other reasons I have seen this happen include, low brake fluid and or really OLD brake fluid. Another possibility is, and this one makes me shudder, a failing ABS booster. So I'll tell you what some of what I know about those.

You can check your brake fluid for moisture absorption with an inexpensive tool:
Brake Fluid Liquid Oil Moisture Tester Pen Car Testing Tool For DOT3 DOT4 | eBay

Opinions vary on this, but if the brake fluid is over 5 years old, you probably should replace it and that gives you the opportunity to disassemble the disc brake cylinder and replace/lube the seal.

About the ABS booster... It is an involved job to change and the pump is not cheap. Last I checked it was around $1500 with tax shipping etc. I tried to find one during the pandemic... I had to call a lot of dealerships to find one in stock. rebuilds on this part (last I checked) were not available. And used units are a gamble. (with your brakes!) There is also a resistor that Toyota recommends be changed with the booster pump and it is located in a hard to reach place beneath the dash board. If you pay a mechanic and he/she does these repairs by the book it will be expensive because it is not a quick job.

How do you know if the booster pump is starting to fail but has yet to make life briefly very exciting? You will hear it running much more often even when you are driving down the road not touching the brakes, coasting with engine off. If you hear it running more than once a minute while just sitting in the parking lot with your foot off the brake, that's a bad sign.
What does that pump sound like? Ah here are a couple times you will hear it clearly:
1) when the car has been sitting "OFF" for quite a while and the doors are closed and you unlock the driver side door and open it. The first thing you will usually hear is this pump bring pressure back to the system: door opens, click (relay to pump), GRRRRR few seconds.
2) when you are done driving and park and shut off the system. You will hear the loop number one coolant pump pushing hot water back to the thermos, then after a while (maybe a minute?) you will hear a sssshhhhh pop GRRRRRRR. That GRRRR noise is the ABS pump running for a few seconds as it pumps the pressure in the brake system down.
Now that you know the sound I mean, sit in the car with the system in ready state and in park. And just listen for that sound. (helps to turn off the local NPR station which is probably playing on the radio) Do you hear it once every few minutes? OK, no biggie. Once a minute? be concerned. More than once a minute? Change the pump. I am pretty sure that if the pump can not make adequate pressure and maintain it during the pre start up and start up cycle you will get a RED TRIANGLE of death and it will not let you drive. (the car is doing that out of self preservation and because it likes you or at least tolerates you, like a cat, because you buy it gas and oil etc)

With all this said, I do not have complete and total knowledge of all things on this car, but I have owned one (2009) and maintained it myself for almost 8 years. And I have helped friends maintain their Gen 2 Prius cars. But check what other people in this forum say about the "barking or grunting brakes" issue.

Questions? Ask me.
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*please note that when not in Australia where there are actual "barking" spiders (of course there and of course they are venomous!) the sound of a barking spider is a euphemism for human or K9 male flatulence because everybody knows "Girls don't fart." (My wife has made this _very_ clear to me because hers smell like pink roses and that is entirely different).

 

Ok, that got me to go listen to the video.

That is a barnyard of clucking hens that appear under the hood for the whole time you hold the brake pedal down, but vanish when you are not braking.

That sound is not from the brake components at the wheels, it is not the pads or the shims or the slide pins or the rubber bits. You can tell 'cause there's no reason that holding a steady pressure on those parts with the car stopped would cause them to make any repetitive motions or noises.

That sound comes from the brake actuator under the hood, which contains small valves and pressure sensors, and a chamber of pressurized brake fluid, kept filled by a pump.

When you're applying the brake, the actuator's job is to open the small valves letting pressurized fluid out to the brakes, until the pressure sensors show the target brake pressure, and then close the valves so that target pressure is held at the brakes, until you let up on the pedal. Then different valves open and let the fluid return to the reservoir, relieving the pressure at the brakes.

Normally, the target pressure should hold pretty steady there while the valves are closed (where would the fluid go?). But if there is a slight leak, either external or internal, the pressure in the lines won't hold steady while you're holding the pedal down. It will drop off slowly, and the car will notice that with the pressure sensors, and continually goose the little valves open for brief instants to send more pressurized fluid, to hold the pressure on target. That's the clucking-hens sound you're hearing.

An external leak is one where fluid really leaves the system. There'll be a wet or drippy area somewhere, and the level in the reservoir may be low. But more often, the problem is an internal leak. No fluid is leaving the car, it's just that not all the valves in the actuator seal off perfectly anymore when they should be closed, and fluid under pressure slowly sneaks back to the reservoir.

For an external leak, you find the external leak and fix it. For an internal leak inside the actuator, you replace that.

In gen 2, there's a single assembly that's the booster pump, the pressure accumulator, and all the valves and sensors of the actuator (ten valves, seven sensors). As you'd guess for anything that complicated, it has a lot of different ways to go wonky, Frequent running of the pump can be the first sign sometimes, and other times you'll notice something else first.

I'll say! When the system's in good condition, you can normally sit in the car with your foot off the brake and read short books between hearing the pump run. Once a minute when you've given it no reason to run is already quite bad.

PriusChat has a lot of wishful-thinking members who like to keep wishing their actuator isn't bad until the pump runs are coming seconds apart. Psychologists can explain that better than car mechanics can.

check.

The brake pump is one-directional: it pumps the pressure in the accumulator up. It never is used to pump pressure down; that will happen naturally because, you know, pressure. Even in a factory-tight system, the fluid under pressure will return to the reservoir in a matter of hours; that's why the pump often runs when you open the driver door and the car has been sitting.

So why does the pump run a minute or so after you park and shut down? The sssshhhhh pop is the sound of the car doing a brake self test; valves open and fluid sssshhhhhes through. Like any other use of the brakes, if the accumulator pressure is near the low trigger point, that self test will cross it, triggering the pump to run and push the pressure back up.

As for the timing "a while (maybe a minute?)", I don't know if it's exactly the same for gen 2, but in my gen 3 the self-test comes 90 seconds after you power off, you can use a stopwatch.

You will get the brake system warning lights on the dash, the braking behavior will be different and not coordinated with regen, and when you read the brake system trouble codes, there are codes to indicate the pressure isn't building to target levels, and that the pump is running longer than it should (often those codes are seen together). If the pressure falls below an even lower target, there will be a high-pitched beep alarm to warn you. Heavy pedal pressure will be needed to slow the car.

 

G'day ChapmanF,
Thanks for the corrections. I thought the release of pressure and pumping after power down was just pressure release and fluid return. I wondered why they did the latter as it seems like it would happen naturally after the pressure release. The reason I thought it was a "pump down" was that the sound sequence was much like what I heard during the "depressurization" step in the brake bleed.

When I did my pump and resistor replacement I also used it as an opportunity to do other preventive maintenance because you do a lot of disassembly to get to that pump and resistor. And I had many of the parts on hand for those jobs and had just been waiting to "git 'round tuit."

Prior to replacing my pump I did the pin lube a couple times and it greatly lessened or eliminated the problem so I assumed that's what it was especially when it happened after some significant rain periods. (yes I replaced the slider pin boot). That went on for years before I replaced the pump. But when I noticed the pump was running while I was coasting down a gentle hill not touching the brake, that's when the alarm bells went off in my head and then I noticed it while parked too. It reminded me of a failing air brake system on a truck.

While the job itself was "non trivial," finding a new pump was equally difficult. Of course I did it during the pandemic.

I just looked at a couple of dealerships and it _seems_ the ABS Actuator assembly and resistor combo are available ~$1200 +tax and shipping. Of course I placed several orders online only to receive cancellation notices after the fact. So I am wondering how available these parts are. I have been worried that these are a critical failure point of the Gen2 if these parts have not come back into production.
If anyone reading this has actually bought one of these pumps lately or had one replaced please chime in and let me/us know if these parts are out there.

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