Hissing, puffing sound when pressing brake pedal

the brake actuators are known to go bad, no trouble lights?

 

is the sound down at the pedal, or in front of you under the hood?

 

Some get a hissing or puffing sound because of gas bubbles mixed in with the brake fluid. When you apply the brakes, more fluid than usual has to flow, to compress the bubbles, before the brakes really move, and you hear that squish sound. Sometimes you also hear it in reverse when releasing the pedal.

The obvious reason there can be gas bubbles in there is if somebody worked on the brakes and had the system open and didn't completely bleed the air out afterward. There's also a less-obvious reason, because there is a pressure accumulator in the system that holds some pressurized nitrogen, and sometimes if that is worn it can slowly leak nitrogen into the fluid. Then you have mystery gas bubbles in there even though nobody had the system open.

Going through the brake bleeding procedure (the full one, including the underhood stuff) is effective for getting the gas bubbles out, no matter how they got in. I recommend it. You don't really want bubbles in there.

If they are there because of work someone did without bleeding properly after, they will probably be gone for good after proper bleeding.

If they are there because of a slow nitrogen leak from the accumulator, you might get to do it again at some future point, but depending on how slow that leak is, it could be good for a while.

 

Has anyone found a true solution to this hissing sound when brakes are pressed?? I have a 2013 prius 109k miles, i started to notice the hissing sound couple of years ago, i ignored it at first as i could not figure out where it was coming from. It feels like its tad bit louder now and i usually notice it at a red light when brakes are pressed. Brakes always work fine, no codes. Since my brake pads are still good i never did any brake service in terms of replacing brake fluid or anything.

 

You are probably hearing the brake booster's hydraulic pump running more often and louder. Rather common as the brake by wire system ages. At some point it will code and the brake booster assembly will need to be replaced.

 

The hydraulic pump running sounds like, well, a hydraulic pump running. Sort of like a rattlesnake, or joy buzzer.

The hissing, puffing sound is the sound of brake fluid rushing extra fast through the valves of the system because bubbles in the fluid require more fluid to flow. Bleeding the bubbles out (see post #5) is a true solution to the problem.

If the bubbles were there because of work done on the system, one proper bleed should take care of them for good.

If the bubbles were there because of slow escape of nitrogen from the system's pressure accumulator, you might eventually get to come back and do this chore again, but maybe not for a year or two. If it comes back a lot faster than that, you might prefer to replace the accumulator.

Having these bubbles present will contribute to the hydraulic pump running more often (because more fluid has to move each time you use the brakes, so the pressure is depleted faster, triggering the pump to run). So, bleeding the bubbles out will extend the life of your accumulator and pump.

You also want the bubbles gone because "brakes always work fine" (despite bubbles) is only true in the normal, computer-controlled operation mode. The valves stay open as long as needed to pass enough fluid to squish the bubbles and make the brakes work. If the system ever drops to fail-safe mode that's not the case anymore, and that isn't something you want to learn the hard way right when your brakes have gone fail-safe for some reason.

If you've never replaced any brake fluid, this bleeding chore will also benefit you by expelling some of the most-degraded old fluid while you bleed, and of course you'll be making that up with fresh fluid.

 

Thank you, i am going to try and have the dealership replace the brake fluid. Lets see if that takes care of the hissing sound.

 

I am going to take your advice and have them replace brake fluid. The sound has been there a while now and the fact that i have never replaced brake fluid could very well be an issue. Thank you for the information.

 

Nothing wrong with replacing the fluid ... you should make sure to have them use the actual "longer bleed" procedure, though. There is a simpler procedure for straight fluid replacement that doesn't even have to use a scan tool, but both actual bleed procedures require a scan tool.

The simpler fluid replacement procedure is not sure to get all the bubbles out, and it's the effect of the bubbles that you're hearing. The "longer" bleed procedure, done right, should get them all out.

 

Man, i just now saw your comment, i did get a bleeding done and it did not fix the hissing problem. This hissing has been there a while, prob like 2 to 3 years now and no code has come up nor has my brakes have acted any different. Do you think i should go back and ask them to perform the bleeding the way you explained it?? And do you still feel this issue is likely still bubbles and air and nothing else??

 

Since you are taking it to the dealer, why not have them check the brake system and find out
where the hissing is coming from????
I can't believe they just bled the brakes and didn't tell you there was a problem....
Now the liability is there's.... They know there is an issue, and didn't tell you?

 

I don't see anything in this thread suggesting the car has been to the dealer yet.

The hissing is the sound of fluid moving extra fast through the system because of bubbles present.

Two days ago I repeated the bleed procedure to get rid of the hissing in mine. Third time I've done so in nine years. There is not a great mystery to it.


Edit: oh wait, somehow I missed post #12. Sorry. Puzzling to still have hissing after you "did get a bleeding done". Was that done recently, or a while ago, and can you say who did it / how it was done?

There are two scan-tool-mediated bleed procedures, a short one for when work has just been done at the wheels or lines, and a longer one for when the underhood bits have been replaced. It takes the longer one to get all bubbles out of the underhood bits. Are you sure they did that?

 

Yes, i told them about the hissing and they said there are no codes present so they cant say for sure if anything is failing, and they also didn't guarantee if the hissing would stop after the brake fluid change. And it didn't. Though the mechanic told me that if this has been going on for sometime and no code has appeared, it doesn't sound like anything too serious.

 

I am sorry brother but i dont know exactly which procedure they performed. I can probably contact them and find out, actually i plan to take it back and use your info to talk with the manager to see if they can do the longer version.

 

Mine has intermittently had spells during which it hisses (or groans, or quacks, or whatever one calls it) when I release the pedal for the past few years. Then it stops for a while. The brake fluid was "flushed" by dealer about 9 years and 94k miles ago. I assume they used the simpler bleeding method.

 

Hi
I just wanted to provide an update, the hissing sound is there even when car is turned off, and when brake is pressed.

I didn't notice that before so i wanted to provide that information. Still no codes and brakes work like they always have.

 

Thank you

 

????
I guess because it's not THEIR car!

Just be aware that the brakes could fail, at any time. You'll still have the ability to stop,
but the POWER part will likely be gone. You'll have to press that pedal very hard to be
able to stop....

But, it's nothing serious....