Brakes lock up after changing master cylinder?

2011 MAZDA 3
88,000 MILES • 2.0L • 4 CYL • 2WD • AUTOMATIC
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MAZDAGUY808
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Changed master cylinder after soft brakes. The problem was fixed and tested around the auto shop, and it seemed great. 15 minutes later on the way home the front brakes locked up and the traction control came on. We cleared the code and drove more to test the issue. It came back, eventually had to remove the ABS fuse to unlock the front brakes and get it home. We don’t know if it’s the ABS control module or not? It seems unusual this would fail right after changing the master cylinder. Any advice?
Apr 23, 2023 at 11:12 AM
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STRAILER
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Yep, it sounds like the master cylinder is defective and the internal plunger is not returning fully which causes the brakes to lock up. Here is a guide to help explain the problem.

https://www.2carpros.com/articles/a-comprehensive-guide-to-fixing-automotive-brakes-locking-up

Here are the instructions on how to change the brake master out correctly and a video to show you how to bench bleed it.

https://youtu.be/WDxvEQrMkBg

Check out the images (below). Please let us know what happens.
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Apr 24, 2023 at 10:25 AM
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MAZDAGUY808
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Also, I guess this makes sense to note. We just purchased this car; the previous owner had changed master recently. We just assumed his master was defective because of the soft brakes that were not fixed with bleeding. So, we changed again bench bled properly, bled at wheels and that solved the problem until on the drive home the brakes locked up.

It seems like the ABS module is a huge problem with these cars, also thinking previous owner changed the master solved the problem and quickly sold the car to us. Do you think it could be the module as well? That’s the conclusion we have come up with.
Apr 24, 2023 at 2:00 PM
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STRAILER
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It could be the brake booster causing the problem, when the brakes are locked up loosen the brake master mounting bolts 3 turns if the wheels start turning the booster is bad. I would try that before doing the ABS module.
Apr 25, 2023 at 8:40 AM