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One Wheel Refuses to Bleed During Brake Bleed

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Mazda3
Hi all, hoping to gain some insight here as I'm at my wits' end with what should be a rather simple brake bleed job for my '08 Mazda3 hatch. I've tried a variety of methods (among other things that I'll mention) but none seem to be providing any results.

The Problem: The driver's rear wheel refuses to bleed. That is, when cracking open the bleeder screw and pumping, no fluid (or air) flows out of the line/caliper. There is no commensurate drop in pedal pressure and height once the bleeder screw is cracked open as well.

What I've tried and done so far: In attempting to fix this problem, I've replaced the rear flex hoses and rear calipers while also verifying that the master cylinder has no leaks and the brake booster builds + holds pressure. There is no blockage in the system, as the driver's rear wheel did bleed correctly a week ago with no issue...it simply refuses to let any air or fluid out now. As mentioned before, I've also tried a variety of brake bleeding methodologies to fix this issue, including:
1. Standard recommended bleeding procedure by starting with the back wheels, then cross bleeding. I've also bled the rear wheels first without cross bleeding.
2. Bleeding the front wheels first (to get any air that might be trapped in the system out the shortest route) and then moving to the back wheels. I've also started with the front wheels first both with and without cross bleeding.
3. Reverse bleeding with a handheld vacuum pump. This was useless and simply introduced more air into the system due to an improper seal.
4. Standard bleeding (rears first, then front in cross bleed pattern) with a Motiv pump affixed to the brake fluid reservoir. This also had a less than optimal seal and did nothing but add some amount of air to the system.

With all this in mind...here's the current situation:
1. As of this morning, I've attempted to bleed all the wheels by starting with the fronts, then rears without a cross bleed pattern. For consistency's sake, each pedal has been pumped 75 times to make absolutely sure there is no air in the system. Because I've bled the brakes so many times, I know the fluid is clean and clear, so it's just being fed back into the reservoir from each wheel via a long rubber tube fitted over the bleeder nipples.
2. Both front wheels and the passenger's rear wheel bleed perfectly; fluid flows healthily through the lines during pedal pumping, pedal feel is good, and the pedal itself depresses while under constant pressure the moment I crack open the bleeder valve.
3. The driver's rear wheel still refuses to let anything through...no fluid, no air, nothing. The pedal feel remains hard (as there is no air being introduced into the system, or trapped in the system itself).
4. Pedal feel is hard as long as the car is off or the key is switched on to battery power with all systems ready.
5. The moment I start the car, the pedal immediately sinks to the floor with very little foot-induced pressure and I can hear an extremely audible "squishy" sound as if there were air in the system.
6. Turning the car off, I can build pressure again as the booster is functional. It is able to maintain this pressure indefinitely.
7. After building pressure again, cracking open the bleeder screws on all wheels show there is still no air in the system, and the pedal feel remains good. Fluid continues to flow under pumping from all three wheels, and the driver's rear wheel continues to do nothing.

So I'm at a loss of what to do here...the car is pretty much undriveable in this condition as there is no brake feel whatsoever.

Is this a proportioning valve issue? ABS unit failure? Vacuum line issue?

I'd love to hear some feedback before I try taking apart the master cylinder and brake booster to see just what the hell is going on.
 
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Mazda3
Update:

After removing all 3 ABS fuses from the fuse box (the 10A, 20A, & 30A) last night and reconnecting the battery, I decided to turn the ignition to "on" under battery power (without cranking the engine). Cracking open the bleeder valve for the troublesome wheel (driver's side rear) and trying to pump still produced no fluid through the line...so I closed the valve and pumped the brake pedal about 20 times total with swift, forceful pumps. I then removed the keys from the ignition, cracked open the bleeder valve again, and tried pumping. This produced some fluid through the line, but not as much as with other wheels that are functional. I decided to turn the ignition to "on" under battery power again, pumped the pedal with the same cadence and pressure as before, and then switched it to off.

This seem to have fixed the issue with fluid flow from the driver's rear wheel.

Cracking open the bleeder valve and pumping now saw fluid flow normally through the line. I must note however that there wasn't much air in the system at all. The pedal still felt firm while pumping. Just to verify that any possible air didn't travel to any other wheels upstream (if it was caught somewhere in the ABS module), I went ahead and bled the other wheels. Same thing, no air, perfect fluid flow, great pedal feel.

However, a new problem seems to have come up...

In the cross circuit wheel from the driver's rear - that is, the passenger front wheel - even though fluid flows normally through the line during pumping, there is a noticeable hissing/squelching sound that seems to come from somewhere near the brake booster/master cylinder/ABS module area. The pedal doesn't feel as tight as the other circuit's wheels (driver's front and passenger rear), and it doesn't feel as tight as the once-troublesome driver's rear wheel. I've pumped the passenger front wheel about 30-40 times in the same method as the others, and can confirm that there aren't any air bubbles passing out through the system.

Here's a video of what it sounds like: PassengerFrontWheel_BrakeBleed_AirSound - Streamable

Sounds like it might be a master cylinder issue? The brake booster itself is able to build pressure nicely when I pump the pedal, and can hold it indefinitely (several weeks+).
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
Hopefully, you did not use the 2-person bleeding method where one person pumps the brake pedal all the way to the floor, multiple times. This method can cause the master cylinder seal to fail from the MC piston being moved further in the cylinder bore than it normally is. If you use the 2-person bleeding method, limit the pedal travel with a wood block or other object.

With ABS brakes and electronic brake control in some vehicles, the EBC module must be energized to cycle its pumps to bleed fluid. If you don't have EBC or an active handling feature that can brake each wheel individually, then you don't need to worry about it. However, I always refer to the factory service manual before doing repairs or maintenance, especially when it comes to brakes.
 
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Mazda3
Hopefully, you did not use the 2-person bleeding method where one person pumps the brake pedal all the way to the floor, multiple times. This method can cause the master cylinder seal to fail from the MC piston being moved further in the cylinder bore than it normally is. If you use the 2-person bleeding method, limit the pedal travel with a wood block or other object.
Unfortunately this is what we did initially, as some tutorial posts/videos didn't make any mention of the risk.

We've since been using a wooden block and continuing to use the 2-person method, and are now about to investigate the master cylinder.
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
Many years ago, I had a brake job done to our minivan. When I picked it up, I immediately noticed that the brake pedal feel was mushy. When I asked the mechanic if he had used the 2-person method, he admitted that yes, they did. Thankfully, he replaced the master cylinder at no cost. The brakes were back to normal, afterward.
Improper bleeding could also result in a mushy brake pedal feel but in this case, the air was completely bled out at the expense of the MC seal.
 
:
Mazda3
Update #2:

I ordered a pair of metal plugs (M12x1.0) for the master cylinder so that way I could do a quick test without having to remove it from the car just yet. Removing the fluid lines from the MC and inserting in the plugs, I decided to go about testing the brake pedal feel in two ways:
  1. With the car switched off
  2. Switching the car on, letting it run for about 5-10 seconds before turning it off.
Testing the brake pedal with the MC plugged and the car switched off produced a tight pedal - though I'm unsure of how tight the pedal should feel (comments/advice would be helpful here). Furthermore, unlike when I had the lines hooked up normally to the MC where the pedal could build pressure via the brake booster and become hard as a rock, it now maintains some level of stiffness, but still has a bit of slop and doesn't build pressure at all. What does this mean?

Onto the test with the car turned on...the pedal was as squishy as ever (would still rise back up, but the same audible air sound and squishiness I've been experiencing throughout this bleed job). Again, it wouldn't pump up to pressure either. Same when I turned the car off, still wouldn't pump up to pressure like it usually did before plugging the MC.

I'd like to point out that in both conditions, the brake pedal didn't "drop" under its own weight, or fall to the floor after applying a set pressure with my foot. The main issue is that it just failed to build any pressure while testing the MC.

Would love to hear any comments/thoughts/feedback/advice etc.
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
I think that points to the MC as the culprit. If you are pressurizing the fluid in the MC and you still have a squishy pedal feel, it is not due to air in the brake lines.
 
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92 MX-3; 18 CX-5
... unless there is air in the master cylinder. It is hard to know where to start to help here. Even if the master is bad every wheel cylinder / caliper should have been able to gravity bleed. Until you removed the brake lines from the master there is no way to get air into it - not unless it is allowed to run dry.
I'm sorry for your situation. But if you replace the master you must first bleed it before moving onto the rest of the circuit.
 
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Mazda3
Currently benchtesting the MC, and it appears that the primary port has a continuous, and steady stream of air bubbles in it that simply cannot be flushed out by bench bleeding. The secondary port, however, is free of bubbles and has excellent, pure fluid flow.

Would this mean that the primary piston seal has failed and the MC should be replaced?
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
A failed piston seal should not cause air to enter the MC. However, there may be a primary port fitting leak that allows air to be pulled into the MC.
 
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92 MX-3; 18 CX-5
I agree with concept: with fluid in the reservoir a bad MC piston seal can't magically introduce air. Check the fitting on the primary port or the tubing you've attached to bench bleed.
It sounds like you are on track to now replace the master cylinder - but that part and all the others you have replaced can't correct the symptoms you had: intermittent spongy brake pedal that occasionally sinks to the floor and after replacing many parts a single caliper with no brake fluid flow.
A bad master cylinder would always give a spongy pedal and the pedal might occasionally sink to the floor - and even require you pick it up. But would still send some fluid to all wheels.
Bad booster would make the brake pedal hard to press. The booster doesn't create pressure. Instead it uses vacuum to reduce the pressure you need on the pedal. Once the car is off the vacuum will leak down and the pedal can seem rock hard. Depending on how it gets vacuum a bad brake booster can also cause drivability issues (vacuum leak) and other diag codes.
I'm assuming that you are certain there are no leaks in your lines or anywhere in the system.
If yes then my guess - really the only thing that satisfies all the symptoms is a bad ABS controller that has a bad solenoid that is leaking and/or is closing a solenoid valve to a single wheel - which is what it does for stability control. A bi-directional diag tool is needed to test and confirm this guess.
 
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Mazda3
Another update: bench-bled the new MC and it seemed to function properly and much better than the old MC (piston built pressure during bench bleeding, no bubbles).

I went ahead and installed the new MC into the car, connected the brake lines and clutch line to it, and proceeded to bleed the brakes again. There were minimal bubbles in the lines, and they were removed pretty quickly. However, the two main problems persist:

1. Brake pedal doesn't build pressure or have good feel when the car is switched on (engine running).
2. Driver's rear wheel still intermittently blocked and fluid will not come out of that line. I can sometimes get fluid out of the line if I leave the car running for 15-20 minutes and then try bleeding that wheel. However, bleeding without doing this (or if I turn the car on and off quickly) will not let fluid through. This makes me think it's absolutely an electronic/ABS issue.

By now, I can confirm that there are absolutely zero leaks anywhere in the entire system. The lines themselves are in good condition, MC is brand new and works flawlessly, brake booster appears to be functioning well, calipers are new, flex hoses are new...

I've also tried bleeding the brakes with the car on and running. Despite the extremely spongy brake pedal, there is solid fluid flow through all three of the wheels that bleed fine, while the driver's rear still doesn't bleed at all.

I'm reaching the end of my wits here. All potential failure points have been addressed, and there isn't much else I can do.

Today I will attempt to check the lines (again), and then see if I can remove the driver's rear brake line from the ABS module so I can place one of my flexible rubber hoses over the requisite port. My thought process is that if we can get fluid out of this port, then the ABS module is working fine and the problem must be downstream. And the opposite would be true if I can't get fluid out of that port.

Does anyone know if Mazda3s have proportioning valves located outside the ABS module and somewhere on the undercarriage?
 
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Pueblo county CO
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CX-5 Sport 16.5 6M
I have heard of weird problems like this could be caused by something like a hose (or elsewhere) getting intermittent blockage from something in the line. A foreign object/material or even the inside of a line delaminating.

I once had a fuel line block from a tiny round piece of material that had been a piece of 'flash' from manufacturing intermittently blocking flow.
 
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Mazda3
I have heard of weird problems like this could be caused by something like a hose (or elsewhere) getting intermittent blockage from something in the line. A foreign object/material or even the inside of a line delaminating.

I once had a fuel line block from a tiny round piece of material that had been a piece of 'flash' from manufacturing intermittently blocking flow.
This might've caused it, yea. Back when I removed the flex hoses they were badly inflamed and quite worn/corroded, so it's possible they caused this.

That being said, prior to doing any brake work the brakes felt quite good (and better than other cars I've driven thus far). This was intended to be a quick, routine maintenance job...
 
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Mazda3
Have you read this. There are some good tips in it but not the part about pushing the brake pedal to the floor.
That's actually the guide I followed when bleeding the brakes originally...but because it didn't mention anything about pushing the brake pedal to the floor...I ended up doing so.

It's one of the better guides out there, but it did miss some critical details.

Anyway...I'll update once I replace the ABS module as that's the only "link" in the entire brake system that is an unknown at this point. Also found a video recently that seems to replicate the exact issue I have:
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
Does your driving route over the years include lots of stopping? My MX5 is an 08 but is not a daily driver + rarely do I drive it when it is raining.
 
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92 MX-3; 18 CX-5
Don't beat yourself up. Every person who has done auto repair (pro or diy) has a sad story about getting beat by some repair or another. Anyone claiming they have never made a mistake or replaced a part that was unneeded is a liar. In retrospect: Was there a test or logical thinking that might have resulted in replacing fewer parts? Understanding the system and testing to confirm parts need replacement is what you want to improve.
- Pushing the brake pedal to the floor while bleeding does not automatically damage the master. It is super common for people to press the brake pedal past normal travel when bleeding brakes and very rare that the master cylinder is damaged.
- Any brake bleed procedure should ultimately read: There is a factory procedure with a specific order of bleeding - and maybe there are some benefits to a specific order, but it does not matter what order you bleed the brake lines if your goal is to simply get air out of the lines.

I kinda agree that at this moment the isolation valve in the ABS controller or the controller itself may be faulty - and maybe you found a known good used part and want to fire the parts cannon again. But consider all the parts already replaced that you now know were good. Maybe they were dirty, rusty and decrepit but definitely not the source of the problem. Personally I would get a wiring diagram for the ABS system and confirm power and ground to the ABS controller and functionally test the isolation valves - to see if some click and some do not. Or, as you considered earlier: disconnect the bad output line from the ABS module while you are not able to bleed one of the wheels to confirm the blockage is at the ABS controller. These tests could be done without a bi-directional diagnostic tool. You will need the wiring diagram and a 12VDC supply.
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
Actually, damaging of the MC piston seal in older vehicles is not as rare as you think. Popular Mechanics magazine has repeatedly warned users not to fully depress the brake pedal when bleeding the brake fluid. This advice is based upon comments by experienced DIY folks and mechanics. I always ask how the fluid will be bled/flushed before this is done at a shop. I used to do my own brakes for many years and quickly learned that there are much better ways to bleed/flush without requiring a helper.
 
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92 MX-3; 18 CX-5
Agree that there easier ways to flush brake fluid, but gravity and a helper to pump the brakes works great. Sure, it's a good idea to avoid bottoming out the brake pedal. IIRC the OP now owns a pressure bleeder so should be in great shape to bleed brakes after replacing the ABS unit.
The issue with the OPs car is that (likely) the ABS controller is at fault. The master and every other part replaced (save pads and rotors - not sure how this job started) were not the cause of the problem and replacing them was unnecessary. My point: No need to be embarrassed, but learn from this expensive and time consuming trial. Understand the system and test to confirm the faulty parts.
 

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