2011 Mazda CX-9 (well used, but well maintained)I've never encountered a brake problem like this. Brakes are engaging and nearly locking up.
So some history.... I replaced the brake booster and master cylinder on this car about 9 months ago and it's been working flawlessly. This morning it was 15 degrees out. Brakes were fine until I stopped for coffee. Was sitting at a drive through and started hearing the dreaded hiss inside the cabin. I figured the booster diaphragm gave out since I replaced the original with a rebuilt one.
I've had brake boosters fail before, but this one is acting very different.
The brakes are engaging by themselves, HARD. I actually have to use my left foot under the brake pedal to pull it up while driving. (I know it's not safe. Don't go there, please). When the car is moving, as soon as I stop holding the brake pedal up, it engages with force - nearly locking up the wheels. And holding the pedal up takes a fair amount of effort, almost like the booster is sucking the rod in. I just went out and looked at it, and there is nothing obviously wrong. No disconnected vacuum lines, etc.
I'm thinking either the booster failed in some really odd way, or a check valve. My car has some kind of check valve inline in the vacuum line going to the booster, and also has a T connection at the booster itself. Mazda calls that an Aspirator, but looks like a check valve.
Any advice? I know what I am doing with a wrench, and I know this car in and out but kinda stumped on this one.
- Weird problem! Do you have any lights or codes?GdD– GdD2025-12-16 15:50:12 +00:00Commented16 hours ago
- No lights or codes. Very strange indeed.David Watson– David Watson2025-12-16 16:59:55 +00:00Commented15 hours ago
- Can you describe the hissing more? Is it constant? Does change when you pull the brake up? Can you trace it under the hood?GdD– GdD2025-12-16 17:03:34 +00:00Commented15 hours ago
- It's constant when the pedal is allowed to move. When I hold the pedal up and drive, there is little to no noise. At a light, it's a constant hiss that will vary depending on the amount of pressure I put on the pedal.David Watson– David Watson2025-12-16 17:19:45 +00:00Commented14 hours ago
- When I parked, and shut the car off, I had to wedge something under the brake pedal to keep the brake lights off.David Watson– David Watson2025-12-16 17:23:14 +00:00Commented14 hours ago
1 Answer1
Given you have to prop something under the brake pedal to keep the tail lights off and the other symptoms it sounds to me like the piston return spring in the brake booster has failed. If the spring was good your pedal would return to the normal position when the engine was off. It's possible the failure of the spring caused some sort of leak which is the sound you hear, it's hard to be certain, it could be the ABS system operating as well.
The only course of action is to replace the booster unit.
- I concur thank you. Just ordered another one. Now I only have to get the car home lol.David Watson– David Watson2025-12-16 18:22:04 +00:00Commented13 hours ago
- @DavidWatson If you have good strong legs and some experience stopping a car without power brakes, you might disconnect the vacuum line to the brake booster and plug or kink and tie the vacuum hose. This would allow you to drive without the death struggle against the brake pedal.MTA– MTA2025-12-16 18:34:46 +00:00Commented13 hours ago
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