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Brake pedal soft and travels to the floor

I have 2008 Mazda 3 with four wheel disk brakes. Quite some time ago I had to make an abrupt stop and since that time the brake pedal has felt soft and the pedal travel has increased, almost to the floor.
I bled the brakes but that did not help, I checked the master cylinder and it does not show any signs of leaking and the fluid level stays constant. I also took the car to brake shop and had them check the brakes and bleed them. They said they bled the brakes, the pads had plenty life left and they could not find any issue with the brake system.
Then one day I had to do a panic stop to miss a red light runner and immediately after that, for the rest of the day, the brakes felt fine, the pedal was hard and did not travel to the floor when braking.
Where is the issue? Everything seems to check out good.

My guess? In the booster.
But it’s only a guess. I’d need to run some tests to try to verify it… and they might very well end up being inconclusive.

However, the next time you get in the car, try this.
Without the engine running, pump the brake pedal. It should become hard and it’s travel should shorten, “topping out” more toward the top.
Then, while pressing on the pedal, start the engine. You should feel the pedal drop and the “feel” become softer.

That is a “quick test” of booster operation. If you don’t feel these things, you may want to disconnect and plug the tube going to the check valve on the booster. If that has no effect on the brake pedal, the booster isn’t working.

Okay, now for the fun part. IF the check valve hung up and is not “unhung”, everything may act normally. That would be the “inconclusive” part of the test.

Post the results.

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Does the car have ABS? You may have trapped a bubble of air in the ABS until and its not getting fully bled out. If I have a spongy pedal after bleeding, I do an ABS stop or 2 a re-bleed.

That assumes you don’t have a wonky booster as @the_same_mountainbik suggests.

It is not a booster issue, that was eliminated the first day.
My thought, like Mustangman, is the ABS.I have heard about hard braking a couple of times and rebleeding the brakes, but I was willing to do that unless someone else had mentioned it.

I don’t think you’ve entirely proved this isn’t a master cylinder problem yet. There isn’t usually a way to visually diagnose if the master cylinder is leaking or not. When it leaks it’s an internal leak, from one chamber to another, so you can’t see the leak. Nothing leaks out, and the fluid level stays the same. Still, that kind of leak could definitely cause the brake pedal to go to the floor. Usually the symptom is more clear than you have. Press on the brake pedal and instead of getting stiff and stopping 2 inches off the floor, if you keep pressing it gradually, slowly, continues all the way to the floor. But they can fail in other ways. It may not leak until you really press hard on the pedal for example. One cause for that behavior is the wall of the cylinder is scored, but only at a spot that affects the seal when you really are pressing hard on the pedal.

I think if I had this problem I’d have a shop check & bleed the ABS with their scan tool. If that didn’t fix it, I’d take a flyer on a new master cylinder. With new fluid and a good bleed, there’s a good chance that would fix it.

Sounds like the master cylinder piston got pushed past its normal operating range and was bypassing internally. Even though the last hard stop appears to have fixed the problem, I don’t think I would trust it to remain fixed.

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Y’know, having read all the posts I’m inclined at this point to agree with the MC suggestion.
NYBo made an excellent point about the MC pistons (you have two on one shaft) having been pushed past their normal range. That can cause problems.

+1
And, it should be noted that a failing master cylinder can act differently, due to differing temperatures. On a very cold day, when internal seals shrink, braking problems may be apparent, but then when temps rise, braking may seem to be normal again.

Does the OP recall if the “bad” day was a really cold one, and if it has been considerably warmer since then?

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If all main components verified good then another possibilityis that the caliper is not getting even pressure at top and bottom when brakes applied. Check caliper for same movement at top and bottom with brakes applied. If not same then check for any loose/missing bolt or rusted caliper slide pin. With caliper off the slide pin should move freely and not sticky or frozen. Have seen this problem on cars with the exact same problem described on this car. Good luck.