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AgnosticRaptor 02-01-2020 03:42 PM

One Wheel Refuses to Bleed During Brake Bleed
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 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'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 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.

AgnosticRaptor 02-02-2020 02:45 PM


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 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+).

theblooms 02-06-2020 11:10 AM

Do you have a scan tool that you can activate the ABS pump with? I'd start there, by cracking open that wheel cylinder, then activate the pump. See if it's pulsing out fluid like you would expect.

lout 02-06-2020 06:18 PM

try loosening the flexible line to the caliper and see if you get fluid there when pumping peddle

AgnosticRaptor 02-17-2020 02:00 PM

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:

With the car switched off
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.

AgnosticRaptor 02-17-2020 02:02 PM


Originally Posted by theblooms (Post 10155678)
Do you have a scan tool that you can activate the ABS pump with? I'd start there, by cracking open that wheel cylinder, then activate the pump. See if it's pulsing out fluid like you would expect.

No, I do not. A while back we tried a $200 scanner from Amazon, and that was absolutely useless. No autostore seems to rent them either.

Got a recommendation for one? I'm currently benchbleeding the MC, but if that doesn't solve the issue I may have to try this.

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