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Hissing sound when pumping brakes?

Tiny
RONALD DEVOLD
  • MEMBER
  • 2006 MAZDA 3
  • 2.3L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 248,637 MILES
While attempting to bleed the brakes, I hear a hissing sound when pumping the brakes. I had this issue since March 2022, when I first purchased the car. So far, I have rebuilt all four calipers and installed new brake pads. I started the engine and let it run for a while and I pressed on the brakes and turned off the engine and the pedal stayed to the floor. I tested for vacuum on the booster hose, and I have vacuum. I checked for leaks around the master cylinder and booster but was unable to determine if air was escaping. I just hear the hissing sound. I don't think it's the booster cause normally it will be hard to press the pedal, but it could be a bad seal somewhere between the booster and master cylinder. But I don't know. I have attached a couple of videos.
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Thursday, January 12th, 2023 AT 3:39 PM

7 Replies

Tiny
BORIS K
  • MECHANIC
  • 799 POSTS
Hello,

A hissing sound from brakes while inside the passenger cabin is a sign of a failing vacuum booster. The vacuum booster uses engine vacuum to increase braking power. The booster uses a rubber diaphragm and a push rod. The rod is connected on one side to your brake pedal and the other side pushes the master cylinder piston.
When the engine is running and you’re not applying the brakes, engine vacuum is applied equally to both side of the diaphragm, so you get no braking assist. As you apply the brakes, a valve opens, allowing atmospheric pressure to flow in to one side of the diaphragm. Engine vacuum then pulls the diaphragm towards the master cylinder, forcing the push rod into the master cylinder.
If the atmospheric valve leaks air, you’ll notice a hissing sound from brakes while you’re inside the cabin.

See image 1+2 below.

Cheers, Boris
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Friday, January 13th, 2023 AT 4:31 AM
Tiny
RONALD DEVOLD
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  • 107 POSTS
Okay. I replaced the booster and master cylinder, but now I am unable to bleed the brakes. I bled each caliper and the brake pedal still goes all the way to the floor.
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Monday, January 16th, 2023 AT 5:02 PM
Tiny
BORIS K
  • MECHANIC
  • 799 POSTS
Hello,

It is always best to pressure bleed a braking system.
You can get inexpensive hand pump operated units, see image below.

How to replace master cylinder:
https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

How to bleed brake system:
https://youtu.be/w7gUsj2us0U

How to use a pressure bleeder:
https://youtu.be/BucnEEe-QEo

Hope this helps.

Cheers, Boris

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Tuesday, January 17th, 2023 AT 3:07 AM
Tiny
RONALD DEVOLD
  • MEMBER
  • 107 POSTS
I purchased a pressure bleeder, and replaced the master cylinder and still the brake pedal still goes to the floor. New booster and master cylinder and still I'm having issues. Could it possibly be the module or the calipers? I rebuilt the calipers also.
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Saturday, February 11th, 2023 AT 9:23 AM
Tiny
BORIS K
  • MECHANIC
  • 799 POSTS
Hello,

You can try reverse bleeding the brakes, from the calipers up to the master cylinder/reservoir.
You can use a large syringe, a fluid pump or I used to use a hand pump oil can, just remove the nozzle and fit over the bleeding nipple.

See a video- tool manufacturer:
https://youtu.be/231oK-_p5so

This is a common practice for bikes:
https://youtu.be/RW1m4_NiGok

Hope this helps.

Cheers, Boris
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Monday, February 13th, 2023 AT 4:03 AM
Tiny
RONALD DEVOLD
  • MEMBER
  • 107 POSTS
I ordered a pressure bleeder. What if back bleeding doesn't solve the issue?
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Wednesday, March 1st, 2023 AT 5:25 PM
Tiny
BORIS K
  • MECHANIC
  • 799 POSTS
Hello,

Brake pedal to the floor can be caused by;
Air in the hydraulic system- in ABS block, master cylinder or calipers.
Faulty brake master cylinder- pressure bypassing seals.
A leak in the hydraulic system.
Expanding brake hoses when under pressure can lead to a spongy brake pedal.

A guide:
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Cheers, Boris
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Thursday, March 2nd, 2023 AT 2:08 AM

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