Hey all, I have a 2010. I'm getting a slight hissing puffing sound when the brake is pressed. Noise goes away when I press the pedal all the way down. It's obviously some kind of leak, is there a known or common issue here? It's really tight down there and would prefer to go straight to the problem if there is a common culprit, thanks!
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.
