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Mazda 3 brakes fading halfway

I have a 2005 Mazda 3, with ABS. Recently, the brakes on this car began to fade about halfway through the pedal travel, then firm up. This is while driving, (ie, with the engine running). When the engine is off, the pedal pumps up high and firm. I bled the brakes, eliminated the possibility of any hydraulic problem such as a leak, no change. I then reasoned that since the problem only appeared when vacuum was applied to the power assist, that must be the culprit. Replaced the power assist, problem remains. Next suspect is the master cylinder, but, as I said, with the engine off, the pedal is fine. Usually cars make sense to me, but this has me questioning my instincts. Any help or guidance would be much appreciated.

Can you describe the “fade?” Use very descriptive things about how stuff feels rather than the term fade. Based on the rest of what you said, it sounds like you’re talking about a soft pedal rather than brake fade.

When you say you replaced the power assist, I assume you mean the booster. If so, did that also include a new check valve and a new vacuum line?

I also have to wonder about how you eliminated the possibility of a hydraulic leak.

I’d assume that if you had an ABS warning light on you’d have mentioned it.

Let’s not over-complicate things here and get back to basics. The master cylinder and power brake booster are the two least likely culprits of this issue, based on your description. The first thing to do is check your base brake components for wear. Worn pads mean more fluid in the calipers, meaning more fluid to compress to get braking action. This can cause a low brake pedal. If your car has rear drum brakes, that is a good place to look for a cause to this problem. Shockingly few people realize anymore that drum brakes are adjustable, and that their adjustment should be checked periodically. Even so-called “self-adjusting” drum brakes can benefit from occasional manual adjusting (and lubricating, as parts can corrode and stick or seize, rendering the self-adjusting mechanism worthless), and drum brake adjustment makes a huge difference in pedal height. I will go out on a limb and say that your description of the problem, to me, sounds like a textbook case of drum brakes that are severely out of adjustment. It would be a shame if you spent hundreds on parts you didn’t need if the problem turned out to be a simple five minute adjustment.

Its your master cylinder. If no other brake work had been done recently and you have not noticed any brake fluid leaks anywhere on the car…Then its your Brake Master Cylinder…They fail more often than you know and the issue you are describing is textbook Master cyl failure…esp if none of the criterea were met in the beginning of my post. No leaks, no brake work done? Then its the cylinder.

Thanks for taking the time to comment!

Good questions. Here are the answers

-The fade feels like this: you step on the brakes and get immediate resistance at the pedal, and the brakes engage. Then, the pedal falls away another inch or so. You must maintain pressure on the pedal (and push through this inch or so of soft pedal) until the pedal stops fading and is firm. Please keep in mind that this only happens with the engine running. With the engine off, the pedal stays put, nice and firm.

-The ABS light has not illuminated.

  • I do mean the booster, but I did not replace the line and check valve. (Also, I forgot to mention, the old booster was making a ‘whooshing’ noise (sounded kind of like an owl), when the pedal was released)

  • I eliminated the possibility of hydraulic leaks by examining the entire system, and finding none.

-The car has 4 wheel disc brakes, and the pads have 3/16 - 1/4 in. material left.

Its possible that this is the first sign of a master cylinder problem.

But I would also check out the check valve & line for the brake booster. The brake booster works by engine vacuum. In essence, the vacuum created by your engine helps you brake by sucking the pedal to the floor. Your engine vacuum varies a lot as you drive. It will dive whenever you hit the throttle (though generally rise & stabilize under steady throttle). If your engine is healthy and you snap off the throttle it will shoot up. The check valve is supposed to help the booster hold vacuum & the vacuum line, of course, provides the vacuum. The valve sort of acts as something of an insulator against the normal fluctuations of engine vacuum - and if it wasn’t doing that well you could very well be getting fluctuations in pedal feel based on the fluctuations in engine vacuum.

That’s my theory anyway. And I’m sticking to it for now - if for not other reason than that check valves & vacuum lines are cheap & easy but master cylinders are expensive and not so easy.

hello. im a auto tech of over 35 years and have read all of these posts about mdigraz brake fade this gentleman is explaining his problem perfectly. i to have a mazda 3 in the shop and at this point have installed three different master cylinders and a power brake booster we have manually bled the brakes per factory spec. we also tried power bleeding and nothing changes. this job started with a master cylinder leaking into the booster . well know we have over come the leak and the noise (moan) inside the car however still the pedal fades only under power and for a second it feels like its going to the floor . i’ve pulled out every trick i can think of . any thoughts please email techshoprepair@aol.com

Sounds like you are the mechanic on the job then?

Aside from any obvious leak in the system OR a failure of the brake calipers/wheel cylinders (all this needs to be ruled out for certain FIRST)…after that…the issue CANNOT be anything else…Unless there is some wierd condition occuring with brake power/fluid somehow going into or being absorbed by the ABS system…but I dont even know what I am trying to describe there so…prob not. REALLY need to make sure this travel is not because of a brake system over-tolerance…esp like can occur in a rear drum equipped vehicle…even damaged brake SHOES can do this…cause excessive travel…but then why would the pedal be firm with engine off…THAT sort of rules out the over-tolerance theory… so you will be back at the master again in my book.

No wait…WITH power assist perhaps this provides the extra force needed to over flex whatever may be over flexible… HOW BOUT…Deteriorated Rubber brake lines blowing up like balloons? Could be there too no? REALLY REALLY need to check all the brake hardware and hoses (internal reinforcement breakage) for flexing or over clearance type issues

After ruling out rubber lines mimicking balloons and any other over tolerance creating Brake HW…I’d LOVE to see you grab a KNOWN WORKING master cyl from another vehicle and try that… Seems like there could be mfg issues with this particular master cyl.

IS THIS A NEW MASTER OR REBUILT? If rebuilt were ALL the masters tried so far form the same rebuilder? The rebuilder might be using the wrong size O rings or seals in the rebuilds and is not aware yet of the issue?

Mark brought up a good point…such as if the rear drum brakes have too much travel to take up…there goes your pedal…same goes for a messed up caliper…Checked them with a critical eye yet?

Blackbird

How about the ABS modulator?

thank for your responce , ive been up and down with this system we have tried three differnt rebuilders with simalier results , i completely agree with alot of what was said however i dont believe the sympton is understood , when bleeding is complete with engine off all is good and tight at the pedal. start and drive all seems good at slow roling stops, excel to about 40 mph then when coming to a stop the pedal starts out tight ,car starts to slow the pedal will suddenly starts to FADE ( like a master cylinder that is leaking internally) then as quickly as it FADDED it stiffens and will stop the car well . all takes place with out removing presure from the pedal . note if when fading you apply one pump to the pedal every thing works great again like a bad master. i am going to look for a balooning of the hoses but unlikley this mazda is a 2005

Hello, i have the identical issue described in this thread(@tech1 @mdigraz) , and was wondering if anyone has found a remedy for the issue. Thanks

If you get no help on this thread, post your problem anew … click “ask a question” from the main forum pages. This is an old thread, so you’ll likely get more help posting a new topic.

I’ve heard of this being caused by a loose wheel bearing. This causes the brake pads to be pushed back into the calipers while driving. Jack the wheel up and see if you can detect any wobble.

If you are not going to repair this yourself just find a good brake repair shop before you crash into someone or something. Brakes are not something to be put off until it is too late.

If all main components verified good then another possibility is the brake caliper is not having even pressure at the top or bottom when brakes applied. Start by checking each caliper as brake is applied for same movement at top and bottom. If not same then check for reasons why such as the caliper slide pin frozen with rust or bolt missing/loose, etc. I have seen this same problem on Hyundai cars with either the caliper holddown bolt loose and Nissan cars slide pin rusted frozen. Good luck

The original poster last heard Sep 2011 , M31 posted 1 time a month ago can we just let this thread die.

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