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Brakes bled again after they have been on a flight?

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Brakes bled again after they have been on a flight?

Old 08-03-19, 02:13 PM
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Bike Jedi
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Brakes bled again after they have been on a flight?

I picked up a new set of Shimano SLX brakeset internationally. The brakes were "pre-bled" to my surprise and I was expecting it all to come in parts and do all the work. Since the brakes have been up and down on a plane since they did this, are brakes subject to pressurization problems? I have never traveled with a bike by plane so I honestly don't know. Or is the system self contained where it isn't subject to the pressurization of the rest of the plane? It wasn't in a cabin and in cargo, so it went through more pressurization stuff I assume, not knowing the science well enough behind it. Anyone want to educate?

Want to know something really cool?? I purposely bought SLX brakes because outside of the cotter pin, the caliber is identical to XT, and for the bang for buck and budgeted kind of thing, SLX is the better route. I was planning on changing the cotter pin out myself and getting the XT pin to upgrade it so that it was XT now virtually (outside of slight weight differnce in handle and one adjustment screw on lever, I now have XT brakes). Well the folks where it came from on Ebay, without advertising it or knowing ahead of time, took the cotter pin out themselves when they put the ICE Tech pads in it, and replaced it with XT screw and cotter pin on the screw just like how the XT brakes are setup, and shipped it that way. I just got my brakes in the mail and it was the first thing I looked at and noticed.

Sometimes it's the little things

So anyway...since the brakes have probably been up and down 4 times by fly by now, do I need to re-bleed them?

Last edited by Bike Jedi; 08-03-19 at 02:21 PM.
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Old 08-03-19, 05:28 PM
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Never Fix A Running Piece. If the brakes work well and have a firm feel to the lever leave them be. If they are not yet installed and you need to shorten the hydraulic lines they will likely need bleeding, although I was able to install my Magura HS33 hydraulic rim brakes without needing to bleed them so it can be done if you are careful.
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Old 08-04-19, 12:36 AM
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Originally Posted by literal trope
It’s a sealed system.
Do you know if that is the correct answer for sure?

I am not a science expert by any stretch of the imagination, and I am not up on my hydraulic skills, but I assume that's what makes it a hydraulic system and why it functions the way it does, because it's a closed system. I don't know what kind of outside pressure can be too much for it to override the system with greater force in some way that my mind doesn't understand.

But I know my toothpaste always splooges out unwanted after flights and I would think that's in a closed system to some degree, but outside pressure overrides that some how and most things I have traveled with in the past like that. I just wanted to be sure that there is nothing I have to be cautious about because I don't understand the science behind all that. I just know my toothpaste container splooges when first opened. Wanted to make sure nothing like that can happen to the brakes because I have never received pre-bled brakes in the mail before.
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Old 08-04-19, 05:06 AM
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Cargo pressure

Fyi, pressure in the cabin in a flight is typically equivalent to pressure at 6000-8000 feet elevation.

Your toothpaste splooges because it isn't bled of air. If there's no gas in a system, there's no chance of gas expanding or compressing. I'd have confidence in my brakes being sealed better than a toothpaste tube.

If you're super concerned, look at the system access points. If you see new oil seepage, maybe the system isn't sealed well enough. Bring along a bleed kit if you're really concerned.

I'd have confidence in my brakes being sealed better than a toothpaste tube.

Last edited by Unca_Sam; 08-04-19 at 05:14 AM.
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Old 08-04-19, 10:13 AM
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[QUOTE=Unca_Sam;21058954]Your toothpaste splooges because it isn't bled of air. If there's no gas in a system, there's no chance of gas expanding or compressing.

"If there is no gas in a system..."

That's the answer I was looking for that will help me understand. Thanks. Now it makes sense why it doesn't happen.

If you're super concerned...
I am not. I just wanted to know and have the knowledge so I know. When you mentioned about that there is "no gas in a system..." then it all clicked and started to make sense. "That is" what I was after...to know what is correct or not, so I know whether I should be "concerned" or not. I never put oil on a chain in my life until a few years ago and I am almost 50. I wasn't concerned. It was because I was ignorant. Was just trying to bring some light on my own ignorance is all. Thanks for being that light.
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Old 08-04-19, 12:43 PM
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I think that your toothpaste oozes out after flight because the tube is getting compressed; likely due to rough cargo handling. Air pressure at altitude is lower than a sea level so it would tend to expand the tube, or the toothpaste or air would leak out during the flight, not after, so would create the opposite effect to what you are seeing.
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Old 08-04-19, 02:35 PM
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Originally Posted by Bike Jedi
That's the answer I was looking for that will help me understand. Thanks.
You're welcome.
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Old 08-04-19, 02:38 PM
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Originally Posted by Bike Jedi
Do you know if that is the correct answer for sure?

I am not a science expert by any stretch of the imagination, and I am not up on my hydraulic skills, but I assume that's what makes it a hydraulic system and why it functions the way it does, because it's a closed system. I don't know what kind of outside pressure can be too much for it to override the system with greater force in some way that my mind doesn't understand.
Hydraulic fluid is non-compressible. If the system is sealed and *fully air-bled, then nothing that will happen to it during shipping will make it leak or introduce air into it.

(other than some hyd oils will expand slightly when subjected to high temperatures)

The changes in ambient pressure and temperature in an airplane cargo hold are nothing compared to the pressure changes that go in inside the brake system itself during operation.

You want to talk pressure? I'm an ROV Pilot, my company specializes in seafloor oilfield construction / maintenance. Typically my subs work at a depth of 5,000-6,000 fsw. Ambient pressure there is ~3,500 psi, give or take. The hydraulic systems that run the subs are functionally no different than anything you'd use on the surface. (other than we have to make sure there's absolutely no air in them)
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Old 08-04-19, 04:17 PM
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Originally Posted by Ironfish653
The changes in ambient pressure and temperature in an airplane cargo hold are nothing compared to the pressure changes that go in inside the brake system itself during operation.
I just needed to hear other's talk about it so I can learn a little. I actually wouldn't have really thought about that until you said this. Makes more sense now in some ways.

You want to talk pressure? I'm an ROV Pilot, my company specializes in seafloor oilfield construction / maintenance. Typically my subs work at a depth of 5,000-6,000 fsw. Ambient pressure there is ~3,500 psi, give or take. The hydraulic systems that run the subs are functionally no different than anything you'd use on the surface. (other than we have to make sure there's absolutely no air in them)
Yes, I am not that talented in any real capacity so that's pretty cool to have skills like that. That's why I like the internet so much because I would normally never get to speak with folks like you who could teach me some basics about hydro in a few sentences.

So this may be a dumb question, but since technically there is no air in the system (assuming a perfect bleed for context), could I take those brakes and put them out the window in space, bring them back to earth, and they will still work? I would guess if it didn't, then we would have never gotten to space in some ways without hydraulic's in some capacity I assume, but I don't know...never really thought about it before. Could I send my bike through space like Elon Musk did with a Tesla car, and when they find the rest of the aliens on another planet, they can get on my bike and ride it on their planet and the brakes will still work?

If so, I want to start a Go Fund Me campaign to put one of my bikes in space with disc brakes. If I raise enough funds, we can probably pay them to do it for us since everything is for sale today.
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Old 08-04-19, 05:20 PM
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Originally Posted by Bike Jedi
So this may be a dumb question, but since technically there is no air in the system (assuming a perfect bleed for context), could I take those brakes and put them out the window in space, bring them back to earth, and they will still work?
I don't see why not. Keep in mind, the seals on a hyd system are there to keep the oil in, not air out. Air pressure (at sea level) is only 14 PSI, so it doesn't take a whole lot to hold that out. Air gets in to a hydraulic system because oil is leaking out, not because air is forcing its way in.
Also, bike brakes are pretty simple.


Now, try to air-bleed this:


Last edited by Ironfish653; 08-04-19 at 05:37 PM.
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Old 08-05-19, 03:13 AM
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Originally Posted by Ironfish653
I don't see why not. Keep in mind, the seals on a hyd system are there to keep the oil in, not air out. Air pressure (at sea level) is only 14 PSI, so it doesn't take a whole lot to hold that out. Air gets in to a hydraulic system because oil is leaking out, not because air is forcing its way in.
Also, bike brakes are pretty simple.


Now, try to air-bleed this:

If they have a youtube video on how to do it, I can do it!

At least in a couple of tries, with unlimited funding for errors, and someone else's awesome set of tools!
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Old 08-05-19, 01:30 PM
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Keep in mind that, even in a properly bled system, brake fluid can have a certain amount of gasses dissolved in it. These will come out of solution as the pressure drops. Try filling a syringe with some brake fluid (my brakes all use mineral oil), then hold your finger over the tip of the syringe and pull up on the plunger. If you pull hard enough, you'll see tiny bubbles coming out of the fluid, eventually forming an air pocket in the syringe. Just google "de-gassing brake fluid" and you should get some hits (including debate on whether it's needed). I don't know if the pressure gets low enough on an airliner for this to be a problem, but it does suggest a potential avenue for air bubbles to form in the brake lines.
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Old 08-11-19, 11:55 PM
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I looked into taking a dog on a plane. The airlines said the baggage compartment has the same environment as the cabin. Many dogs travel in baggage. Even champion dogs the are going to a show.
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Old 08-12-19, 11:52 AM
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Also keep in mind an airplane is filled with hydraulics, if it were an issue, they wouldn't use it on a plane. The brake lines on your bike are sealed, unless you experience a problem, I wouldn't give it a second thought.
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Old 08-13-19, 12:24 AM
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It should be fine. If it feels good, it is good.

If you deceased ambient pressure on a hydraulic brake with significant air in it it would probably just expand the bladder in the resevoir slightly, slightly extend the pistons, or leak out of the reservoir. None of which are big deals.

Also the number of above posts that do not understand that there is a reservoir in a hydraulic disc brake... The system is only really sealed when the brake lever is depressed because the piston in the master cylinder moves past the opening to the resevoir. The reservoir is necessary because the pistons advance with pad wear and the additional fluid needs to go somewhere. Also the resevoir is usually the highest point in the system so air tends to collect there so it can do no harm.

Last edited by cpach; 08-13-19 at 12:29 AM.
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Old 08-13-19, 06:35 AM
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Originally Posted by merlinrkd
I looked into taking a dog on a plane. The airlines said the baggage compartment has the same environment as the cabin. Many dogs travel in baggage. Even champion dogs the are going to a show.
The pilot needs to know if there are live animals in the cargo hold, and then activate cargo heat. Otherwise, the cargo hold does not get heated up (saving fuel in the process) The temperature then falls significantly - you can actually tell on a longer flight if cargo heat was on or not by the temperature in your luggage right after you get it. The pressure in the cargo hold is the same as in the cabin.
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Old 08-13-19, 11:31 AM
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Originally Posted by cpach
It should be fine. If it feels good, it is good.

If you deceased ambient pressure on a hydraulic brake with significant air in it it would probably just expand the bladder in the resevoir slightly, slightly extend the pistons, or leak out of the reservoir. None of which are big deals.

Also the number of above posts that do not understand that there is a reservoir in a hydraulic disc brake... The system is only really sealed when the brake lever is depressed because the piston in the master cylinder moves past the opening to the resevoir. The reservoir is necessary because the pistons advance with pad wear and the additional fluid needs to go somewhere. Also the resevoir is usually the highest point in the system so air tends to collect there so it can do no harm.
The reservoir is also sealed to ambient air pressure. What you are describing has nothing to do with changes in air pressure. If the bike is upside down, at just the right angle, for long enough, and the lever is pulled, there is a slight possibility that air could get into the line. Chances are though, there will be no one in the cargo hold to compress the brake lever.
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