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Old 08-29-2019, 10:48 AM   #1
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Spongy Brakes

Haven't been driving stop and go for quite awhile but yesterday took the van into PDX and ran a bunch of errands on a hot day with tons of stop/start. Breaks started out just fine but as the day progressed they got spongier and spongier. By middle afternoon when I stepped on the brakes the pedal would go down considerably. If I pumped it just once it came back up but would start sinking with continued pressure.

Master Cylinder? Any help appreciated.
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Old 08-29-2019, 11:06 AM   #2
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I'd start with a fluid flush. You might just have too much moisture in the fluid, and as your brakes get warm, the steam will flash off and it'll feel just like there's air in the system.
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Old 08-29-2019, 01:51 PM   #3
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It might seem obvious, but have you checked the brake fluid level?
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Old 08-29-2019, 03:12 PM   #4
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It might seem obvious, but have you checked the brake fluid level?
Also, if it's low, then either your pads have worn substantially, or you have a leak. Either way, just topping off without investigating the other bits is something I'm no longer willing to do.
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Old 08-30-2019, 08:20 AM   #5
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Check the brake lines for any cracks, see if there’s any fluid seeping. If you replace, look at getting stainless braided lines made up, nice upgrade and will give you better peddle feel.
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Old 08-30-2019, 11:48 AM   #6
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When my master cylinder went on my 07 SMB 6.0 ~70k miles (leaving camp 200 miles from home) the brakes went rock hard and the AC stopped working. Lovely warm drive home with a lot of planning when it came to braking.

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Old 08-30-2019, 12:15 PM   #7
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Thanks for the input... ya'all're great. Fluid level is good. I'll get a flush and see where I'm at. I'll post results
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Old 08-30-2019, 12:20 PM   #8
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In pre ABS days, a failing master cylinder would slowly sink to the floor when applying steady pressure to the brake peddle. Pumping the brakes would often time return normal function until the next use. Now, with ABS I don't know if that still holds true.
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Old 08-30-2019, 12:59 PM   #9
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In pre ABS days, a failing master cylinder would slowly sink to the floor when applying steady pressure to the brake peddle. Pumping the brakes would often time return normal function until the next use. Now, with ABS I don't know if that still holds true.
Yup. And same with a small leak, such as when you slap on new calipers, and you accidentally leave the old crush washer stuck to the brake hose, and throw a new one too.
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Old 09-05-2019, 11:20 PM   #10
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I had a master cylinder fail that way on my Honda. It was, uh, interesting on hills. The pedal would slowly sink until it hit the floor, then I'd have to use the handbrake to hold the car while I pumped it back up. Needless to say I replaced that ASAP.

The general rule of thumb is if the pedal sinks and you're not losing fluid, it's probably an internal master cylinder leak. The fluid can leak around the piston seals without actually escaping the system.

If the pedal's spongy but doesn't sink when you have your foot on it, you may just have air or moisture in the system.
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