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I've got a 2008 Mazda 3i with 120k miles on it, manual transmission. The brake pedal started going halfway to the floor before the brakes engage, although braking action seems normal after that, maybe a little soft pedal. Brake fluid level looks good, no dash lights are on. I suspect the master cylinder is going bad, but could it be anything else?

This started a week or so ago, after the first snow of the season. When it snows, I like to test the brakes / road by braking hard from 10 or 15 mph after pulling out of the driveway. This time it engaged the ABS (it usually does), so I'm worried it's more than (just) the master cylinder.

Edit: brakes were fine before my hard braking test. Rear brakes were replaced 500 miles ago, front brakes are about half worn.

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    If any air is in your brake lines, that'll also cause it, plus each time you take the cap off a little bit of moisture builds up in the fluid, If the fluid was checked multiple times, over time the brake pedal goes further and further down until you bleed the air pockets and moisture build up out, bleed the furthest caliper bleeder screw first because it'll draw out the most fluid.
    – user38183
    Nov 26, 2018 at 12:19

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you have lost fluid any number of ways such as

1) if the pads become worn, there is more space for fluid inside the caliper, and this can cause it

2) The master cylinder could be leaking or the fluid can evaporate in desert climates

3) the system is so worn it is damaged and leaking onto the ground or the car

4) There could be air in the system from an above leak.

5) the pedal is hard when the system is full and sealed, so now it is not full and sealed and from ground to firewall yuu have the brake disc, brake pads, calipers, ABS module, steel lines, master cylinder, pedal assembly

6) the part that is not obvious is the tiniest bubble in the system can screw it up

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  • My brake fluid reservoir is full; no appreciable amount of fluid has been lost
    – mmathis
    Nov 26, 2018 at 4:21
  • Did this come on all of a sudden, or has it been progressively degrading over time?
    – DavidRecallsMonica
    Nov 26, 2018 at 4:31
  • @David it was a sudden change. Added an edit to my question
    – mmathis
    Nov 26, 2018 at 11:58
  • @mmathis I suspect the master cylinder seal has failed, allowing brake fluid to pass from the "working" side of the piston to the back side. Such failure is consonant with the age of the vehicle. If you're handy, you may be able to rebuild the m/c; if that's not your cup of tea, replace it. The brake system will need to be completely bled afterwards, to purge it of air.
    – DavidRecallsMonica
    Nov 26, 2018 at 15:18
  • once we had to bleed an old dump truck for 4 hours before the pedal got hard. Bleeding an old system can take a very long time, and it seems like nothing is getting better as you keep going. Imagine pumping up the brakes, yell "ok", while your friend opens the valve, and it squirts out, then do it again for 4 hours. on the last time, it gets hard.
    – steve
    Nov 26, 2018 at 15:28

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