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Discussion Starter #1
Hi,

I've recently gotten back into MTB and decided to learn to bleed brakes last week. Got all the parts, looked up the Shimano guide, all went fine.

But I have a question - why are MTB brakes bled "backwards," as in why do we push fluid from the caliper up to the reservoir? When you bleed almost any other brake system you do it by:


  1. Over-filling the reservoir
  2. Opening bleed nipple on caliper
  3. Depressing brake pedal/lever (pushing old fluid out bleed nipple through a tube to a catch container)
  4. Closing bleed nipple
  5. Releasing brake pedal/level (pulling new fluid from over-filled reservoir)
  6. Repeat 2-5 til the fluid is clear

The reason I ask is that I noticed I picked up quite a lot of air when I filled the syringe... I got most of it out but could still see a few little air bubbles in there no matter how hard I tried to get rid of them. These may have ended up being pushed into the system which is the exact opposite of what we want when we bleed brakes, and I can't think of a really good way to get rid of these 100%.

Is there a reason MTB brakes are bled the opposite way to anything else, or is it just that the syringe system is a bit easier and maybe designed so anyone can do it?

Cheers
 

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You're free to push/pull fluid in either direction, including back and forth while bleeding. The goal is to get all the air out. If you get all the air out, it doesn't matter which direction(s) of flow you used. You do want to get all the air out of the reservoir too, as that eliminates the possibility of any of it getting in the system. If you hold both syringes vertically with plungers up, any air that comes out of the system will rise in the syringe so that when you push fluid in either direction, you're not introducing air into the system. You need to exercise caution if you choose to apply vacuum with the syringes as it can draw air into the system through imperfect connections between the bleed hoses, lever or caliper.
 

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Discussion Starter #3
You're free to push/pull fluid in either direction, including back and forth while bleeding. The goal is to get all the air out. If you get all the air out, it doesn't matter which direction(s) of flow you used. You do want to get all the air out of the reservoir too, as that eliminates the possibility of any of it getting in the system. If you hold both syringes vertically with plungers up, any air that comes out of the system will rise in the syringe so that when you push fluid in either direction, you're not introducing air into the system. You need to exercise caution if you choose to apply vacuum with the syringes as it can draw air into the system through imperfect connections between the bleed hoses, lever or caliper.
Sweet that's what I thought, just wasn't sure if there was some special reason MTB brake bleeding systems work from the caliper up.

I did think about taping the syringe to the fork/rear triangle to keep the plunger up and let bubbles settle that way but bleeding the old school way is what I'm used to so might be easier to just do that. I think I can still use the syringe to catch the fluid being pushed out (the system will just push the plunger out when the lever is depressed), and there shouldn't be vacuum near it if I'm closing the bleed nipple before releasing the lever.

I'm much more confident in getting a good bleed by working from the res down so might spend this arvo re-bleeding that way :)

Thanks
 

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The Shimano brakes on one of my bikes is the easiest.

I just lever bleed once in a while to get rid of the air in the lever.

Maybe once a year I'll flush the automotive way. I meant to do it when I replaced pads a few months ago but that project has gotten away from me. Maybe over this long weekend I'll find time for that project.
 

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Hi,

I've recently gotten back into MTB and decided to learn to bleed brakes last week. Got all the parts, looked up the Shimano guide, all went fine.

But I have a question - why are MTB brakes bled "backwards," as in why do we push fluid from the caliper up to the reservoir? When you bleed almost any other brake system you do it by:


  1. Over-filling the reservoir
  2. Opening bleed nipple on caliper
  3. Depressing brake pedal/lever (pushing old fluid out bleed nipple through a tube to a catch container)
  4. Closing bleed nipple
  5. Releasing brake pedal/level (pulling new fluid from over-filled reservoir)
  6. Repeat 2-5 til the fluid is clear

The reason I ask is that I noticed I picked up quite a lot of air when I filled the syringe... I got most of it out but could still see a few little air bubbles in there no matter how hard I tried to get rid of them. These may have ended up being pushed into the system which is the exact opposite of what we want when we bleed brakes, and I can't think of a really good way to get rid of these 100%.

Is there a reason MTB brakes are bled the opposite way to anything else, or is it just that the syringe system is a bit easier and maybe designed so anyone can do it?

Cheers
Agreed, been bleeding brakes on motorcycles for years and never from the caliper up. The think with some is that the bubble rise in the system, floating to the top, therefore you want to not fight that and push them out at the top.

I'm with you though, pressure bleeding is the way to go. My real issue is finding a way for the drain tube and catch bottle to not keep falling off the caliper nipple.
Where's the hack for that?
 

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Sweet that's what I thought, just wasn't sure if there was some special reason MTB brake bleeding systems work from the caliper up.
It varies from mfr to mfr, honestly.

Yeah, Shimano is primarily a caliper > lever bleed process. It's dead easy, which is nice. I think for Shimano, simplicity was something that they really aimed to achieve in the bleed process.

SRAM and many others operate with a syringe on both ends of the system, so you wind up pushing fluid both directions throughout the bleed process.

My Hayes Dominions go this route, and add another wrinkle. The calipers have two bleed ports. So not only can I do the typical lever-caliper bleed (pushing fluid both directions), but I can also push fluid (and bubbles) from one side of the caliper to the other to clear the air out.
 

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I have found the most consistent and air-free bleed is without any syringes. Just fill and attach the fluid cup, and open up the bleed screw on the caliper until you see no more bubbles. (Pads out and bleed block installed of course) Tighten the bleed nipple, and flick the lever a couple times until any remaining bubbles exit into the cup. Works every flippin time.
 

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I have found the most consistent and air-free bleed is without any syringes. Just fill and attach the fluid cup, and open up the bleed screw on the caliper until you see no more bubbles. (Pads out and bleed block installed of course) Tighten the bleed nipple, and flick the lever a couple times until any remaining bubbles exit into the cup. Works every flippin time.
So basically you are doing a gravity bleed vs. a forced bleed? I know that with Shimanos you also have the ability to "burp bleed" the system (what one shop called it). They just opened the screw on the lever and then started tapping the system lines/caliper/lever and you could see small bubble pop at the lever screw. Did this for a little bit, put a couple drops of oil on the screw port, did some more tapping to ensure bubbles were not introduced into the system and bam felt like new! Wish SRAM would get their act together.
 

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I've done the gravity bleed, which is good for expelling the fluid in the caliper that has gotten hot and discolored. But whenever I put a vacuum at the lever, more bubbles come out. It seems either way works ok. I think Shimano brakes are tolerant of a little air in them.
 

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So basically you are doing a gravity bleed vs. a forced bleed? I know that with Shimanos you also have the ability to "burp bleed" the system (what one shop called it). They just opened the screw on the lever and then started tapping the system lines/caliper/lever and you could see small bubble pop at the lever screw. Did this for a little bit, put a couple drops of oil on the screw port, did some more tapping to ensure bubbles were not introduced into the system and bam felt like new! Wish SRAM would get their act together.
I have found the "burp bleed" to be hit or miss. If you've ever seen the inner workings of the servo wave levers and piston orifices, you'll see why any air bubbles may not exit properly. The "gravity bleed" uses the velocity (albeit slow) of the fluid to help pull the bubbles out. It has always worked for me.

And, I've seen a lot of guys applying way too much pressure with the syringe method which causes cavitation (boiling) when the fluid is forced through the tiny internal orifices.
 

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I've done the gravity bleed, which is good for expelling the fluid in the caliper that has gotten hot and discolored. But whenever I put a vacuum at the lever, more bubbles come out. It seems either way works ok. I think Shimano brakes are tolerant of a little air in them.
If you apply too much vacuum, you can "boil" the fluid at ambient temperature. Thus the bubbles you see. It doesn't take much with a heavy thumb. LOL.
 

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I have found the "burp bleed" to be hit or miss. If you've ever seen the inner workings of the servo wave levers and piston orifices, you'll see why any air bubbles may not exit properly. The "gravity bleed" uses the velocity (albeit slow) of the fluid to help pull the bubbles out. It has always worked for me.

And, I've seen a lot of guys applying way too much pressure with the syringe method which causes cavitation (boiling) when the fluid is forced through the tiny internal orifices.
I hang my bikes vertically against the wall, and have found 'burp bleeding' to be very effective for removing air bubbles that collect at the lever over time. I don't seem to be the best at bleeding brakes, and I find that doing a burp bleed the day after I do a full bleed (with the bike stored vertically) gets rid of any bubbles in the system that I might have introduced or missed.
 

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I think Shimano brakes are tolerant of a little air in them.
They are, because in use the design moves air up into the reservoir above the business part of the system. So you can have a little air in there but pull the brake a few times and it'll work. Basically, if you keep topping off the reservoir and riding the bike, eventually you'll end up with a perfect bleed!
 

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Absolutely. Another method I've used is to prop your bike at a 45 degree angle, install the fluid cup (with fluid in it), and keep it open over night. A few light taps on the M/C, and a couple flicks of the lever the next morning before reinstalling the reservoir screw, and you're good to go.
 

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If you apply too much vacuum, you can "boil" the fluid at ambient temperature. Thus the bubbles you see. It doesn't take much with a heavy thumb. LOL.
Well I did an experiment with the syringe and clear tube that had bubbles stuck to the side and really small bubbles suspended in the fluid. When I applied a vacuum, they moved to the top of the syringe. I did not observe any boiling. I cannot find a T-v diagram of mineral oil, but maybe that is because it is not a pure fluid.
 

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Well I did an experiment with the syringe and clear tube that had bubbles stuck to the side and really small bubbles suspended in the fluid. When I applied a vacuum, they moved to the top of the syringe. I did not observe any boiling. I cannot find a T-v diagram of mineral oil, but maybe that is because it is not a pure fluid.
It doesn't boil, but I can see how someone might think so; as you apply a vacuum, the suspended air in the solution expands, allowing you to effectively extract more of it. Almost all oils have some air in suspension, and most cases on a bicycle do benefit, however little, from having the air 'extracted' from solution.
 

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They are, because in use the design moves air up into the reservoir above the business part of the system. So you can have a little air in there but pull the brake a few times and it'll work. Basically, if you keep topping off the reservoir and riding the bike, eventually you'll end up with a perfect bleed!
love this about my shimanos....bike sideways in truck for hours, lever has zero feel, few quick lever pumps, ready to ride
 

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It doesn't boil, but I can see how someone might think so; as you apply a vacuum, the suspended air in the solution expands, allowing you to effectively extract more of it. Almost all oils have some air in suspension, and most cases on a bicycle do benefit, however little, from having the air 'extracted' from solution.
You do realize the definition of "boiling" is when the vapor pressure of a liquid equals the external pressure acting upon it, right? Heat is one way to achieve that (and it is what we are most familiar with), but pressure (or vacuum) will achieve the same results.
 

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Low pressure will achieve the same but I am still skeptical of your claim without seeing the numbers. The question is, what pressure will mineral oil boil at 295K and how easy or hard is that to achieve with a $2 syringe.
 
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