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Hydraulic Disc brakes problems

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Hydraulic Disc brakes problems

Old 09-23-20, 06:38 PM
  #1  
msdumo
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Hydraulic Disc brakes problems

I'm the fix it guy in my neighborhood and even though I don't have a bike with hydraulic disc brakes I have had 3 people with fairly new bikes that have lost brakes this summer.

I bleed them and all is good. I have a theory. They transport or store them upside down horizontally in a garage or on a roof car top carrier.

My theory is when a bike is upside down and if there is air in the reservoir which might be low on fluid, the air makes its way up to the caliper because of the upside down position.
Air in the caliper results in a brake loss. Bleeding the brake corrects the problem.

Does my theory hold water?
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Old 09-23-20, 06:53 PM
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Yes.

I only have one bike with hydro's but I've heard this for years.

I'm sure some brands are more affected than others but you're on the right track.

I use the same upside-down hooks but add a loop of rope to them to allow upright hanging. Probably not a big deal for a quick flip but if stored a while, worse.

Have you even laid a bottle of mineral oil on its side, it will definitely leak. Doesn't matter how tight the cap is on. How are the seals on the levers much different?
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Old 09-23-20, 07:15 PM
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Originally Posted by msdumo
Does my theory hold water?
Yes, but you shouldn't need to give them a full-on bleed every time. Orient the bike in a stand such that the brake line is constantly running uphill from the caliper to the lever, so that there's no trap for the bubbles to get stuck in. Give the lever quick, vigorous flicks. Most of the time, this'll be enough to get the air back up in to the reservoirs in the levers (air in there is normal).

And tell them to stop putting their bikes upside down (the Shimano manual specifically mentions this).
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Old 09-23-20, 07:43 PM
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I hope not. Water would be bad.

Joking aside... Yes any air in the system would migrate to the high point. In normal orientation the high point is the caliper lever. Any air is supposed to migrate back to the lever after a few pumps, where it's vented behind the piston into the reservoir by ports.

Hydraulic brakes are pretty simple, arguably simpler than cable disc brakes.
https://www.servicearchive.sram.com/..._brake_v06.pdf
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Old 09-24-20, 09:59 AM
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Originally Posted by Darth Lefty
Yes any air in the system would migrate to the high point. In normal orientation the high point is the caliper.
Huh? Isn't caliper the thingy that holds and activates the pistons that push the pads against the rotor? They are the low point on an upright bicycle. Normal orientation is upright bicycle, no? I guess you mean an upside down bike...
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Old 09-24-20, 10:21 AM
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Yup, and very common with Shimano rear brakes. An easy test is to lift the front wheel up so that the bike is nearly completely vertical. Engage the rear brake, if it is non functional or very soft, that is a classic sign you need to do a quick burp and fill. Very easy to do and takes minutes.
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Old 09-24-20, 10:26 AM
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Originally Posted by Chuckles1
Huh? Isn't caliper the thingy that holds and activates the pistons that push the pads against the rotor? They are the low point on an upright bicycle. Normal orientation is upright bicycle, no? I guess you mean an upside down bike...
You are correct, I miswrote
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Old 09-24-20, 03:09 PM
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if the hydraulic system is properly bled there should be no air in the system and air will not find its way into the system unless there is a leak. a properly bled system should be virtually maintenance free unless you decide to open the system to change the hydraulic fluid.


I think your theory is full of holes, or the bikes hydraulic system was bled poorly from day 1
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Old 09-24-20, 04:02 PM
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I had Avid Juicy brakes that did this if you flipped the bike over to work on it or something. But then it would recover (to its usual lousy state). My XT and Ultegras don't do this.
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Old 09-24-20, 08:03 PM
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Originally Posted by superpletch
if the hydraulic system is properly bled there should be no air in the system and air will not find its way into the system unless there is a leak. a properly bled system should be virtually maintenance free unless you decide to open the system to change the hydraulic fluid.


I think your theory is full of holes, or the bikes hydraulic system was bled poorly from day 1
^This^ And of course this post shouldn't be in GD, it should be in 'bicycle mechanics'.
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Old 09-24-20, 09:04 PM
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my jaguars would be mushy for a few squeezes. if you slayed it down or worked on the bike it but it seems normal for them.
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