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post #1 of (permalink) Old 02-01-2020, 03:42 PM Thread Starter
AgnosticRaptor
Bicycle Rider
 
Join Date: Jan 2020
Location: Midwest
Posts: 5
Unhappy 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 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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