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Hydraulic disc brake fail?

Old 03-14-22, 08:20 PM
  #1  
dstke
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Hydraulic disc brake fail?

Hi,


My wife and I were riding down a steep hill slowly, braking hard. Once the angle subsided I released both brakes coasted for a few seconds, when I tried to reapply the brakes the right side rear brake lever went straight to the handlebar with absolutely no effect. I stopped the bike using the front brake and initially there was no resistance to right lever pulls but slowly over time pressure built up, even to the point where when we continued the ride it felt like full braking power. A little disconcerting. Anyone else ever experience this?


Setup: SRAM RED etap brifters and brake calipers, relatively new pads (disco sintered steel), Magura MRP 203mm rotors
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Old 03-14-22, 09:22 PM
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I have never experienced total failure. It is usually preceded by the brake feeling a bit spongy. It sounds like that brake needs bleeding.
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Old 03-15-22, 05:45 AM
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We run the the same brifter/calipers on our CoMo, but have never experienced what you describe. Towards the end of last season the rear became spongy so I added some fluid but did not do a full bleed. Over the next few rides it got worse so I did a full bleed with new fluid and it’s been fine since.

I would second Brent’s recommendation.

how long of a descent were you braking? We ride in a hilly area, however, descents rarely reach a mile in distance. Could your brakes have overheated?
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Old 03-15-22, 04:11 PM
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I have had a similar experience, except we were going 45 MPH (on my electronic speedometer) on a 10% hill when we needed to stop quickly.

Both rotors were smoking hot. If you had fade from overheating it will be a long decline that you were trying to go down slowly by using the rear brake or a panic stop from high speed with a heavily loaded tandem.

If you could smell the hot brakes it could be water that was absorbed out of the air by your brake fluid, or a low boiling point fluid.

Either way, flush the fluid and replace it with fresh fluid. Castrol SRF is quite expensive, but it has the highest boiling point of any DOT fluid I am aware of. This will also result in any air in the lines being removed if the job is done properly.

If the fluid is high quality and not contaminated, you might get larger calipers that will work with your SRAM setup,,(Hope makes some). They will make that Castrol SRF fluid look cheap.

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Old 03-15-22, 08:07 PM
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I never brake consistantly on front rear or both .
It is best to cadence brake by releasing one for a few seconds reaply then release the other for same time reaply etc. this gives the disc and pads time to cool and keeps the whole process safe.
Also the brake fluid should be flushed out with new fluid every 3 to 4 years on the coast, can extend to 4 to 5 years inland where the humidity is lower.
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Old 03-16-22, 07:03 PM
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Thanks for the advice. I will bleed the brakes. I have been using the recommended SRAM DOT 5.1 brake fluid. I switched from SwissStop Exotherm2 brake pads to DiscoBrakes sintered steel pads after a pair of SwissStops wore out in about 500 miles last Fall. The Exotherms have heat sinks and may help preventing the pads/caliper/line/fluid from overheating. I just ordered another couple pair and hope that the pair that gave out so quickly were a one off.
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Old 03-16-22, 07:13 PM
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Dot 3, 4 & 5.1 brake fluids absorb water. When overheated, water turns to steam and will allow the lever to go to the bars without any braking effect. Once the fluid cools, steam converts back to water and normal braking is regained as water won't compress. Overheat the pads, calipers and the fluid to waters boiling point and it will happen again.

In cases like this, a complete fluid flush with a new bottle of brake fluid is in order.
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Old 03-16-22, 08:24 PM
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Originally Posted by nomadmax
Dot 3, 4 & 5.1 brake fluids absorb water. When overheated, water turns to steam and will allow the lever to go to the bars without any braking effect.
Actually steam expands exponentially and will lock up your brakes by barely touching the lever.

Old cars will do this. Gently applying the brakes limits the brake caliper temp. But applying the brakes with force will quickly heat up the fluid/water to steam and lock up the tires - so instead of stopping - the vehicle slides right through the intersection!

Sounds like you have an air pocket that moved from the master cylinder past the piston to the brake line. Then later moved back into the master cylinder. Either way, a brake bleed is in order.
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Old 03-16-22, 09:06 PM
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Originally Posted by masonv45
Actually steam expands exponentially and will lock up your brakes by barely touching the lever.

Old cars will do this. Gently applying the brakes limits the brake caliper temp. But applying the brakes with force will quickly heat up the fluid/water to steam and lock up the tires - so instead of stopping - the vehicle slides right through the intersection!

Sounds like you have an air pocket that moved from the master cylinder past the piston to the brake line. Then later moved back into the master cylinder. Either way, a brake bleed is in order.
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Old 03-16-22, 09:30 PM
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Originally Posted by masonv45
Sounds like you have an air pocket that moved from the master cylinder past the piston to the brake line. Then later moved back into the master cylinder. Either way, a brake bleed is in order.
That is what I was thinking. I have a mountain bike that used to have Avid Jucy's (the technology SRAM purchased for their brakes). I turned the bike upside down to adjust something in a campsite, flipped it back over, and presto -- no brakes! A couple hours later sitting upright, II tried it again, and it was fine.

I ultimately fixed the problem by getting XT brakes.
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Old 03-16-22, 10:47 PM
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Originally Posted by masonv45
Actually steam expands exponentially and will lock up your brakes by barely touching the lever.

Old cars will do this. Gently applying the brakes limits the brake caliper temp. But applying the brakes with force will quickly heat up the fluid/water to steam and lock up the tires - so instead of stopping - the vehicle slides right through the intersection!

Sounds like you have an air pocket that moved from the master cylinder past the piston to the brake line. Then later moved back into the master cylinder. Either way, a brake bleed is in order.
I hate to be that guy but........you're wrong. Look it up. Steam compresses and will cause a soft or spongy lever or pedal just as I described. It's also the reason why when the fluid cools and the steam goes back to being a liquid it will behave normally. I won't bore everyone with the links, unless I have to. It's an important subject and people should know the correct facts, so I can't let this one stand.
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Old 03-17-22, 10:07 AM
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Can you message me the links? I don't mind being wrong - but would like to know why. 🙃
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Old 03-17-22, 11:13 AM
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Originally Posted by masonv45
Can you message me the links? I don't mind being wrong - but would like to know why. 🙃
What Is Brake Fade & What Causes It? | Tires Plus

Brake fade - Wikipedia

Or about 100 other sites if you google "brake fade".

It's like I said in the beginning, all glycol based brake fluid is hygroscopic, meaning it attracts/absorbs moisture from the atmosphere. Once it's sufficiently contaminated with water, the boiling point of the fluid is no longer what the spec was on the bottle, it's now 212F, the boiling point of water. It doesn't take a lot of hard, repeated applications to get there, the heat is generated by the pads/rotors and transmits to the pistons which in turn transmit it to the fluid. The water in fluid, once above 212F is converted to steam/gas and WILL COMPRESS unlike water (below 212F). That compression will result in a spongy pedal or lever, sometimes going all the way to the bars or the floor without providing adequate braking, or even none at all. Once the brakes and fluid cool, the water vapor/steam turns back into water and the brakes will operate normally because water DOES NOT compress.

This kind of brake fade won't happen with a cable operated disc brake because there's no fluid creating leverage on the pads. They can experience pad fade, which is something completely different. To put a fine point on it, your statement that water turned in to steam in a brake line will lock up the brakes is the exact opposite of what happens. School is out, I'm gonna go ride my bike with rim brakes
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Old 03-19-22, 08:28 PM
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Originally Posted by masonv45
Can you message me the links? I don't mind being wrong - but would like to know why. 🙃

not to pile on, but piling on, water in brake fluid makes brakes fail. Driving a car on the track, water in brake fluid can lead to total brake failure and planting into the wall.

same thing can happen on a bike if you get water in brake fluid hot enough to boil. When you put the brakes on, you compress the steam, instead of moving the pistons.
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Old 03-21-22, 10:36 AM
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Old 03-27-22, 10:46 AM
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+1 on the replacing the brake fluid, and make sure to use a fresh bottle as the one on the shelf in your shed that you used 4 years ago, will have absorbed water too.
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