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What could cause this hissing sound when the brake pedal is pressed?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by SB6, Jun 26, 2020.

  1. SB6

    SB6 Member

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    Hopefully the link works. I'm talking about the hissing sound, not the buzzing from the brake booster.

    I had my brake actuator replaced by an independent shop a few months ago, using a used brake actuator I bought off of eBay. Could a faulty actuator or a bad installation cause this sound? I've noticed it since the replacement, I think. Might just be in my head, but I feel like stopping time has been increased too. The car will slow down, but going from maybe 15ish mph to 0 takes longer than I feel is normal, and I'll end up nearly flooring the brake pedal to make sure I stop in time.

    I don't really know much about the brake booster system, but could the noise be some kind of leaking vacuum line? Is that a thing?

    I've recently replaced the front brake pads, and more recently, I flushed my brake fluid.
     
    #1 SB6, Jun 26, 2020
    Last edited: Jun 26, 2020
  2. ChapmanF

    ChapmanF Senior Member

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    Not so easy for me to listen to the imgur links, but if it is a whooshy sound when you're applying or releasing the brakes, there is probably still air in the system that didn't get completely bled out. Air is compressible, so when you apply the brakes, more pressurized fluid has to whoosh from the accumulator into the air-filled spaces and compress the air; when you release, the air pockets expand again and whoosh the fluid back into the reservoir.

    I would suggest the long-form bleed sequence in Techstream (the one suggested for after replacing the actuator).
     
    #2 ChapmanF, Jun 26, 2020
    Last edited: Jun 27, 2020
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  3. SB6

    SB6 Member

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    That's what I had suspected... I do have a mini VCI cable, but I've never been able to correctly set up Techstream thus far. Every time I try, Techstream is missing functionalities like the "Utility" button and the check system health function.

    But as indicated by my other thread, my water pump seems to have just gone out, so I guess I have a more important problem to deal with before this :(
     
  4. Raytheeagle

    Raytheeagle Senior Member

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    What version of Techstream do you have :whistle:?

    I had version 12.something and it didn't have the functionality that version 10.30.028 did:cool:.

    I went back a couple of versions and the functionality was there:).

    If you don't need the newer version based on the age of your Prius, I'd go back a couple of versions and see if that helps(y).
     
  5. SB6

    SB6 Member

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    Hmm sounds like I'll need to look that. I have a copy that says 12.something as well as a copy that says 13.something. Had the same missing functionalities issue with both.

    I plan to more fully look into it after I solve my water pump issue. I foresee it taking time, as info about Techstream seems to be a mess, sort of all over the place :unsure:
     
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  6. Sonic_TH

    Sonic_TH Active Member

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    Did you fixed the water pump?
     
  7. SB6

    SB6 Member

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    I did. Why?
     
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  8. Sonic_TH

    Sonic_TH Active Member

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    Just to know, wanted to know the update.
     
  9. SB6

    SB6 Member

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    I know this is an old thread, but I never ended up addressing this issue. I would like to do so soon. To reiterate the issue, there's a hissing or whooshing sound when I press the brake pedal, the brake booster buzzes and re-accumulates pressure quite often, and coming to a complete stop requires me to depress the pedal further than I would normal expect. Also, when I'm braking to a complete stop, once I get down to around 8-12mph there is sometimes a feeling of the car jerking forward. I believe I read that's about when the car shifts from regenerative braking to the mechanical/friction braking?

    Some things I believe the issue may be: 1. Brakes or brake booster/ABS system need to be bled/flushed. 2. Brake booster ("booster assembly", pump/actuator assembly, or both) are bad. 3. Something else?

    Before this, I had an issue with my brake booster and ended up replacing just the "brake booster assembly" and not the pump assembly. I replaced using a used part from eBay, but it was apparently the updated (or rather, superseding) part, as it was listed with "mark" #47210-47310, which is part #47050-47140. The actual part came did have 47210-47310 on it, so it did seem to be the newer version. The replacement was done by a local independent shop. I really don't know if the independent shop did the proper bleeding after replacing the brake booster assembly. I did want to try doing the bleed myself, but haven't been able to get Techstream running correctly yet. The last time I tried setting up Techstream, I was missing a lot of functionality, for example the "utility" button.

    Some time after the brake booster assembly was replaced, I was working on the front brakes and accidentally had the front right caliper piston pop out. I put it back, but I was warned that there could be a leak in the piston boot now that the piston had come out once. However, I have not noticed any loss of brake fluid, so I doubt there is a leak there.

    I did later do a brake flush, but that was my first time ever doing a flush or even a bleed, so I will be redoing it, as I may have done it incorrectly.

    Any ideas?

    Many online parts sections of Toyota dealerships in the southeastern region of the USA currently have a 25% off promo going on right now, up until midnight tonight I believe. It looks like I can get the brake booster accumulator and pump assembly at less than $900 total currently, so I'm tempted to just buy brand new brake booster parts to give myself peace of mind that my issue isn't with the brake booster parts. Maybe sell my current parts after to recoup some of the cost, as they still seem to be okay, no codes currently. Thoughts?
     
    #9 SB6, Mar 23, 2022
    Last edited: Mar 23, 2022
  10. ChapmanF

    ChapmanF Senior Member

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    Referring to the picture, can you be specific about which item (or both) you are looking to purchase?

    Without the picture, the names often get confused.

    [​IMG]
     
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  11. SB6

    SB6 Member

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    Apologies, I did get them mixed up.

    I'm thinking of buying both. Like I had mentioned, when I previously had my brake booster issue (thread linked in my last post), I only replaced the "brake booster assembly" (not the pump), and the replacement was a used part from eBay, although like I said, it was the newer (superceding) part number. At the time, my specific codes had to do with the master cylinder and linear solenoid valve, if I'm remembering correctly.

    Currently I don't have any codes. The issues I do have, as like I said in my previous post, are the whooshing sound, the brake booster tries to accumulate pressure/buzzes pretty often, and I end up having to press the brake pedal harder than expected once I get down to 8-12mph
     
  12. ChapmanF

    ChapmanF Senior Member

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    If there is air that hasn't been bled from the system, that can be a simple explanation both of the whooshing sound and of the frequent pump operation.

    Air bubbles are compressible, so when you apply the brakes, a larger volume of fluid has to whoosh out of the accumulator to compress the bubbles, before the target fluid pressure is reached. Then, when you release the brakes, those bubbles re-expand, and drive that larger volume of fluid whooshing back to the reservoir.

    Because that uses up a larger volume of fluid with each use of the brakes, the pump also has to run more frequently to make up for it.

    So, if it were happening to me, I would go carefully again through the long-form bleed procedure and see if that improves anything, before taking more drastic measures.

    It's possible that an air-bubbles-and-whooshes situation can fail in an undesirable way. In an older car, with significant unbled air, you might not be able to slow or stop the car at all: you can only press the brake pedal so far, and if it is still just squeezing bubbles when it hits the floor, you're out of luck.

    In the Prius, the pressure accumulator normally masks that feeling. You might notice the extra whoosh, but the fluid pressure target still gets hit, and the car does stop. The risk, though, is what happens if the system ever detects a problem and switches to "fail-safe" mode. In that mode, they effectively are an older car's ordinary brakes, with your foot on the pedal supplying the pressure, and once again if it hits the floor just squeezing bubbles, that's all you've got.
     
  13. SB6

    SB6 Member

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    Any chance someone can point towards a Techstream setup tutorial that worked for them? I'm currently poking around trying to set it up again, so that I can perform a bleed, but haven't had much luck with getting Techstream to work so far.

    I have a 64-bit Windows 10 laptop, but will be running Techstream on a virtual machine. I currently have a 32-bit Windows 7 virtual box as well as a 64-bit Windows 11 virtual box installed.
     
    #13 SB6, Mar 23, 2022
    Last edited: Mar 23, 2022
  14. SB6

    SB6 Member

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    Nevermind! Managed to get Techstream working properly for the first time ever! Will post more details later, but for now the version I got working was 12.00.127.

    Gotta go get stuff ready to try a brake bleed!
     
  15. SB6

    SB6 Member

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    So... I failed to bleed properly...

    For some reason, all the vinyl tubing I have is the wrong size and won't fit over the front wheel bleeders or the stroke simulator bleeder. I had standard airline tubing, as well as some tubing that came in a cheap auto part store brake bleeding kit that I found to be worthless. I was able to get the tubing to fit over the rear bleeder nipples, but not the front bleeder nipples. Any suggestions on what size/what kind of tubing I'd need?

    Also,the ECU kept disconnecting for some reason at the stroke simulator bleeding step. Any ideas why? Is the master reservoir level sensor supposed to be disconnected during the entire process? In the beginning it says to disconnect, but I haven't seen it say to reconnect yet, although like I said, I haven't gotten past the stroke simulator part.

    Just adding that I was trying to do the bleeds using the one-man bleed method where you keep one end of the hose submerged in brake fluid in a container, keep the hose raised above the container and bleeder valve in the middle, and keep the other end connected to the bleeder valve.

    I would have likely had better results with better fitting tubing, as well as if I had a partner. Will try.
     
    #15 SB6, Mar 25, 2022
    Last edited: Mar 25, 2022
  16. Matt98svt

    Matt98svt Junior Member

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    Did you ever get this resolved? I am having a similar problem. Just replaced the accumulator and booster and doing the bleeding process with techstream, works great until I get to the stroke simulator part and it says ECU disconnect error. It does not seem to be pushing any fluid out of the stroke simulator when I press the pedal
     
  17. ChapmanF

    ChapmanF Senior Member

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    As I mentioned on your other thread, I had a lot of headaches like that with one dongle I was using, and not with a different dongle I used later (on the same laptop, same Techstream version, same car).

    Mine was disconnecting at a different spot; it would usually get nearly all the way through, then disconnect during the half-dozen zero-downs/pump-ups that happen at the very end. (That's a super-frustrating place to have it flake out, too.)

    Can you think of anything special about the stroke-simulator bleed step in your case? Is it taking you extra time to get around front and do the stroke-simulator stuff? Sometimes I wonder if some dongles just have extra-short timeouts coded into their drivers, and flake out if they haven't seen something happen in that amount of time.
     
  18. Matt98svt

    Matt98svt Junior Member

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    I cant really think of anything other than its doing it almost immediately after i press the button to bleed it. It makes that electrical click like its sending the command and then the error. I will restart techstream a few more times and see if i can make any progress before calling it a night
     
  19. SB6

    SB6 Member

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    No, I haven't been able to fix it yet, unfortunately...
     
  20. Tbkilb01

    Tbkilb01 Active Member

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    Look at accumulator volts on most scan tools
    Steady voltage on 30 second window...find culprit with live data...