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I had brake drums/rotors replaced, along with shoe adjusters in June 2021, and it's now 2023. Not sure how many miles we've put on them, but we've driven from Arkansas to Arizona, Montana, and more since then and the brakes have been perfectly fine - no issues.

Two days ago I went through some relatively deep mud - not axel deep, but definitely tires-to-the-rim deep - and suddenly my brakes have been exceedingly spongy. I stopped and looked under the truck for any visible leaks and there wasn't anything obvious - just normal dirt/brake dust caked around all four brakes. None of it appeared wet or oily. The brake master cylinder had plenty of fluid - at the max line - though it's pretty dark colored. I thought perhaps there may have been mud or other liquid or maybe some debris so I tried a couple of hard braking events but that didn't change anything.

When stopping, the brakes will provide some resistance at first, then slowly (but not slowly enough!!) sink to the floor. Pumping the brakes will provide some moderate resistance with each pump, and I don't detect any failure for the brake pedal to return. Just as soon as you press it it will feel about like normal and then just give maybe 20-50% normal feel, until it bottoms out, and the truck slowly stops.

If the car is off, pushing the pedal produces a "wooshing" sound (the brake booster?), and pumping the brakes with the power off will actually firm up the brake pedal. Turning the ignition on and pressing the pedal will return to the spongy brake feelings. Then turning the ignition back off and pumping the brakes will woosh again a few times until the pedal firms up.

I haven't completely re-checked the fluid level since returning home (I checked initially on level ground and I don't have anything level available at home), so it's possible that it was just a small leak that I couldn't see - would a helper pumping the pedal be useful, or is there something else that's obviously wrong with my brake system here?

My next steps:

  • Found a video that suggested pulling the vacuum(?) line off the brake booster under the hood and listen for a "woosh". If it's missing, will probably need to replace the booster? (Edit pulling the vacuum line off the booster had a very distinct "woosh" sound, so it appears to be holding pressure)
  • Get a helper to pump the brakes while I'm looking around under the car for leaks
  • ???

Is there anything else I should try/do here?

Further diagnosis

I ended out having to drive another trip and an interesting fact: the brakes appear to be stable. That is, if you press hard and fast, and then do not press, simply hold the brake pedal where it stopped brake pressure remains applied. Like, on a hill if you come to a stop, the brakes will keep the vehicle stopped -- it doesn't appear that pressure is leaking from the system. Just slower/further pressure will let you depress the brakes further.

At one point, braking on completely dry ground it sounded/felt like the ABS system momentarily (maybe a half second) kicked in. That symptom never repeated. I had a helper pump the brakes with the engine both on and off and was unable to see a leak anywhere near the wheels, I didn't completely follow the brake lines underneath so there may be something there. I don't know if there's any evidence of water in the brake fluid - it all appears a uniform dark/black color. I did come across a single YouTube video mentioning that for some reason their ABS module(?) actually failed in the wrong way (in the video it sounded like the way it's supposed to fail will simply not produce ABS, but I guess it may have failed in such a way to let pressure out). I do have the money light/check engine light on -- though when I checked it was the O2 sensor code before, and it may have another code that I haven't checked.

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In order:

  1. Check that there is no air or water in the system. A flush and bleed will probably take care of this.
  2. Check for bulging rubber brake lines. These wear out over time and lose their ability to hold against pressure. On a vehicle this old this is quite possible.
  3. Check for fluid leaks at the wheels and in the lines between the master cylinder and the rest of the sytem.
  4. Could be a worn master cylinder. A replace or rebuild may be in order.
  5. Could be that the power brake vacuum system is not working properly. A clogged or collapsed vacuum line or a leaking diaphragm in the booster.
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    I agree with all but the booster being at fault. I can’t see how that would allow the brake pedal to sink to the floor. Maybe I am misunderstanding?
    – HandyHowie
    Feb 16, 2023 at 17:31
  • I did check the rubber portions of the brake lines at the front - no apparent bulging. Does water mix with the brake fluid? It wasn't creamy or foamy at all like oil would be, and there's no separation in the master cylinder. I think it was relatively new/replaced within the last 5 years (at least it's way shinier than surrounding metals) Feb 19, 2023 at 19:04
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    Water doesn't really "mix" but brake fluid is hydroscopic which means it readily absorbs water from the air. The generally accepted max water content is 3% but some manufacturers may specify a different value. Check the water content and see if it's too high. 5 years is a VERY long time between flushes.
    – jwh20
    Feb 19, 2023 at 19:44
  • Would a brake leak always just spew fluid out? We changed the fluid (it's been around 2 years since we had our rear brake drums, shoes, and cylinders replaced - I know we did our front brakes but not sure if the rotors were also replaced then - they do look fine tho) - I never did hear or find an obvious leak, and the fluod exchange definitely improves the brakes. But it may still be possible to continue bottoming out the brakes - just way harder. I never tried before, since I just stopped pressing when I stopped. Should it just be firm, or impossible to bottom the pedal? Feb 24, 2023 at 16:37

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