Brake bleeding - done it but still don't get it.

fastar1

fastar1

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So I have some conceptual problems trying to understand how brake bleeding works.

The first problem is, how does all the old fluid get replaced with new fluid if the bleeder valve is right next to the brake line (as it usually is)? Won't the new fluid just run directly from the brake line to the bleeder valve without circulating through the caliper itself? This is supported by my experience getting my front brakes bled when I wanted the fluid flushed and refilled. The lever is a bit stiffer now but it's nowhere near as good as it was when the SS lines were first installed.

I'm also perplexed with how air bubbles are removed. If anything in the system was opened up there should be significant air in there. Seeing as how air is lighter than oil, the bubbles will tend to rise yet the bleeder screw is at the bottom of the system, so it shouldn't be possible to get all the air out unless bleeding is done faster than the air can rise through the lines. I've never hurried to bleed brakes, and I've never seen anyone rush it either.

The way I look at it, the only way to really bleed brakes properly is to flush and refill the system. The calipers would have to be removed and allowed to hang with the bleeder valve at the lowest point, and the fluid should all drain out. Then to refill, the calipers should be hung in the air, above the master cylinder so that 100% of the air is removed.

Obviously nobody does this, so what am I missing here?
 
fastar1 said:
So I have some conceptual problems trying to understand how brake bleeding works.

The first problem is, how does all the old fluid get replaced with new fluid if the bleeder valve is right next to the brake line (as it usually is)? Won't the new fluid just run directly from the brake line to the bleeder valve without circulating through the caliper itself? This is supported by my experience getting my front brakes bled when I wanted the fluid flushed and refilled. The lever is a bit stiffer now but it's nowhere near as good as it was when the SS lines were first installed.

I'm also perplexed with how air bubbles are removed. If anything in the system was opened up there should be significant air in there. Seeing as how air is lighter than oil, the bubbles will tend to rise yet the bleeder screw is at the bottom of the system, so it shouldn't be possible to get all the air out unless bleeding is done faster than the air can rise through the lines. I've never hurried to bleed brakes, and I've never seen anyone rush it either.

The way I look at it, the only way to really bleed brakes properly is to flush and refill the system. The calipers would have to be removed and allowed to hang with the bleeder valve at the lowest point, and the fluid should all drain out. Then to refill, the calipers should be hung in the air, above the master cylinder so that 100% of the air is removed.

Obviously nobody does this, so what am I missing here?
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pressure pushes the old fluid out, a little mixing happens at first but eventually all the old fluid will be flushed out under the pressure... Just watch the fluid coming out, when its all the same new color its been flushed...
Air rises in fluid, so your bleeders should be at the highest point in the calipers and master cylinders, but again once you get some pressure in the system...Your pushing the bubbles out no matter where the nipple is located....Moving calipers and masters around so the nipple is at the highest point in the system can help, even tapping them can help loosen up bubbles stuck in the system...

And then you have vacuum bleeders that suck fluid and air, while your pushing it as well. Sometimes it just takes time and patience to get all the air out...And other times it goes real fast, every system is different

If your brakes are still spongy and require any kind of pump up before getting full pressure, then you still have air in the system, or possibly a leak...Air compresses, fluid does not...So what is happening there is your brakes are soft untill you pump them up compressing the air, then they get hard, when you let go the air decompresses again leaving your brake soft untill pumped up again...When a system is fully bled properly, the lever should be as stiff the first pump the same as it will be after 100 pumps...
 
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to add to Johnnys post, you can get the bubbles to get out of the system with the bleeder valves at the bottom through two ways.

actually crack the banjo bolt at the master while squeezing the lever and bleed it there. this is usually one of the highest points in the system. or the racers trick is to ziptie the brake lever to the handlebar over night. this allows the fluid to rise to the top and escape to the reservoir.

the second way worked for me when i was bleeding my clutch.
 
Here is what u do. Drain the old fluid, put the new in, new will push the old through bleeding process and u should get bulk of air in 5 min or so during this, then open reservoir cap squeeze brake lever and wrap it with elastic to handle bar and leave it over night, this will push the rest of tiny air bubles from system.
Hope it helps
 
Listen, as I said I understand how to bleed brakes. I just don't understand how it can work given the way the system is laid out, and the properties of air/fluid.
 
fastar1 said:
Listen, as I said I understand how to bleed brakes. I just don't understand how it can work given the way the system is laid out, and the properties of air/fluid.
Click to expand...

Whats not to understand you have a master cylinder that acts like a pump, and pushes the fluid. When the bleeder is closed that moves the pistons in the calipers out. Open the bleeder and instead it pushes air and fluid out out of the bleeder. Push the fluid and air out un till there is no more air, opening and closing the bleeder appropriately to let air/fluid out but not back in... ..Pretty damn simple to understand IMO.....
 
I explained my confusion in the first post. Let me try again...

For one thing, getting the old fluid out and replacing it with new fluid is like trying to replace the stagnant water in a pond with fresh water by pouring it in one side and draining it out the other. Sure, eventually the pond will be full of fresh water but it will take tons more volume of fresh water than merely the volume contained in the pond, as well as some delay for the old water to mix with the new, and even then it will never be entirely free of old stagnant water. So how does this seem to work for bleeding brakes?

My other problem is that when it comes to removing air from the brake system it's like trying to get the air out of a barrel by filling it at the top and draining it at the bottom. Won't work, the air will remain in the barrel. Why does it seem to work with brake systems?

Capisce?
 
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Pump it out of your reservoir before filling with new fluid. Get most of it out till it's almost empty then add new fluid.
Then pump brakes and open bleeder while on brakes, close bleeder and pump again. Repeat this process till only clear fluid is pumped out of the system but make sure reservoir dosnt go empty.
It's that simple.
There are other ways of doing it to but I find this one the easiest to explain.
The air bubbles will simply be forced out of the line by the pressure, if done rite there shouldn't even be bubbles unless a brake component was replaced like a line or caliper.
The fluid in the lines is like a congo line, and the bubbles will pretty much come out with the bleed process. The bubbles won't just float immediately to the top because brake fluid if far thicker then water.
 
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fastar1 said:
I explained my confusion in thew first post. Let me try again...

For one thing, getting the old fluid out and replacing it with new fluid is like pouring fresh water into a stagnant pond and draining the other side. Eventually the pond will be full of fresh water but it will take tons more volume of fresh water than merely the volume contained in the pond, as well as some delay for the old water to mix with the new, and even then it will never be entirely free of old stagnant water. So how does this seem to work for bleeding brakes?

My other problem is that when it comes to removing air bubbles it's like trying to get the air out of a barrel by filling it at the top and draining it at the bottom. Won't work, the air will remain in the barrel. Why does it seem to work with brake systems?

Capisce?
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Ok so think of it this way, yes you can take apart the system...drain the old fluid, clean everything really nice. Put it back together and start with fresh fuild...Now youve wasted a ton of time, and because your starting with a dry system you will use just as much fluid and most likely even more then if you just flushed it out in the first place...So what is the point? Average fluid is $10-20 a bottle, the fluid I use is $60. My time required to clean out the system is worth more then even a full $60 bottle of fluid, and bleeding from complete dry takes even longer!.....So like most people I just flush it out till its clean...Were talking about maybe 10% of a bottle of fluid required to flush out all fluid...

Your not just filling with fluid, your pushing it with the master...And if you have a vacuum bleeder you are also sucking it at the same time...So no its not just like filling a a barrel at the top and draining it at the bottom....Air bubbles get stuck in some spots, but will eventually find its way to the bleeder...Being at the highest point helps, but isnt required...There are many tricks to help coax bubbles to finds its way to the bleeder...For example if you having alot of trouble open the bleeder and push the pistons in the caliper all the way in....Sometimes that can help force any bubbles stuck in the caliper to the bleeder...Or I will even take the bike for a quick spin to loosen up bubbles, and come back for a quick re bleed...

Its not rocket science, but if youve come up with a better idea....By all means enlighten us...Ive been bleeding my own brakes with a piece of clear tubing for 8 years...Never had a problem...I also have a $300 vacuum bleeder and I rarely use it, the old school clear tubing method works just fine...
 
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i get what you're saying and yea, you never fully bleed all the old fluid out by just pumping new fluid in. it gets diluted with the new stuff....
 
sounds like he doesnt know about the bleeder cap/nipple on each caliper..pumpup...loosen nipple....tighten nipple..pumpup...etc
 
The OP clearly stated that he knows the procedure; it just isn't clear to him why it works.
And let's face it, pushing bubbles down a tube (instead of up) is counter-intuitive.

I'm no expert, but this is my understanding:
Between the high viscosity of brake fluid, and the narrow tubes of the brake system, the bubbles tend to 'stick' rather than float as they might in a barrel. (think of the bubbles in a glass of ginger-ale... some stick to the sides of the glass, and even moreso in the narrow straw). Because of this, bubbles do get pushed down.
As far as flushing the old fluid, again the narrow channels prevent too much mixing from occuring; the little bit of mixing that does occur is considered acceptable, and it does not need to be a perfect flush.

At least that's what I figure.
 
if you get bubbles in the system (like i did) what you can do is zip tie the brake lever so it's engaged and then let the bike sit over night. also vibration helps too, it just so happened when i had put the bubbles in the system i was about to trailer the bike from one place to another so i clamped the brake lever shut after it was loaded and the short ride (1 hour) on the trailer bounced the bubbles out.
 
Any brake caliper that I have ever seen has the bleeder nipple at the to(if installed correctly), there is no way you will remove air from the system with the bleeder at the bottom of the caliper
 
I was confused when I read that too - maybe the calipers are on backwards? Bleeder should be at the top.
 
he is not talking about bleeding the bubbles awaway.

hes talking that if the caliper is below the nipple (like it is) and the new fluid arrives at the top of the caliper as well (like it does) by what force does the old fluid leave the caliper and new fluid gets pushed in. by conventional physics the old fluid would never leave the caliper. thats whats confusing to him.
 
jolomatic said:
The OP clearly stated that he knows the procedure; it just isn't clear to him why it works.
And let's face it, pushing bubbles down a tube (instead of up) is counter-intuitive.

I'm no expert, but this is my understanding:
Between the high viscosity of brake fluid, and the narrow tubes of the brake system, the bubbles tend to 'stick' rather than float as they might in a barrel. (think of the bubbles in a glass of ginger-ale... some stick to the sides of the glass, and even moreso in the narrow straw). Because of this, bubbles do get pushed down.
As far as flushing the old fluid, again the narrow channels prevent too much mixing from occuring; the little bit of mixing that does occur is considered acceptable, and it does not need to be a perfect flush.

At least that's what I figure.
Click to expand...

This is it.

Air bubbles will rise, but with the narrow passages in the brake line and the viscosity of the brake fluid they rise very slowly. Pressurizing the system by pumping the brake lever then opening the bleeder on the caliper will flush the air bubbles down much faster then they are are able to travel up the brake lines. Once in the caliper, the passages are much larger and fluid viscosity does not have the same effect. The air bubbles will rise to the top of the caliper, typically above the banjo fitting, and will be expelled when the bleeder is opened.

Red_Liner740 said:
he is not talking about bleeding the bubbles awaway.

hes talking that if the caliper is below the nipple (like it is) and the new fluid arrives at the top of the caliper as well (like it does) by what force does the old fluid leave the caliper and new fluid gets pushed in. by conventional physics the old fluid would never leave the caliper. thats whats confusing to him.
Click to expand...

It will never fully flush out the old fluid, this is true, but just by the random molecular movement of a fluid the old and new fluid will mix. As more new fluid is pushed in via the brake line the old fluid will become more diluted. The old and new fluid WILL mix, from what i've experienced bleeding brakes it does not take much fluid to effectively dilute the old fluid enough to be "new enough" again. This leads me to believe that the fluid capacity of the calipers is relatively low. If the caliper only holds 10-20mL of fluid, by the time I have pushed 50mL of new fluid through it the fluid that is exiting the bleeder is very clean. Sure there will still be some old fluid in there but at that point the juice is no longer worth the squeeze. The benefit of further dilution or disassembly of the brake system is hardly worth the effort.
 
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As you suspect, I doubt that bleeding the brakes will remove all the old fluid. You just dilute the amount of old fluid in the system. The lines and master cylinder will have new fluid but the caliper itself will still have old fluid inside. If you bleed the brakes after installing new brake pads, the cailper pistons will be fully retracted and less fluid will be in the caliper, thus less old fluid will be left in the system.

As for getting air out of the system, the motion of the fluid moving through the lines will push the air into the caliper where it can exit the bleed screw. If there is any air left in the front brake lines, it will eventually rise through the lines to the master cylinder and exit the system, so long as it is not trapped in a high point in system.
 
caboose483 said:
It will never fully flush out the old fluid, this is true, but just by the random molecular movement of a fluid the old and new fluid will mix. As more new fluid is pushed in via the brake line the old fluid will become more diluted. The old and new fluid WILL mix, from what i've experienced bleeding brakes it does not take much fluid to effectively dilute the old fluid enough to be "new enough" again. This leads me to believe that the fluid capacity of the calipers is relatively low. If the caliper only holds 10-20mL of fluid, by the time I have pushed 50mL of new fluid through it the fluid that is exiting the bleeder is very clean. Sure there will still be some old fluid in there but at that point the juice is no longer worth the squeeze. The benefit of further dilution or disassembly of the brake system is hardly worth the effort.
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Thanks, that makes sense unfortunately.

I wish I could get my front brake lever as stiff as it was when the fluid was replaced last year but it sounds like I'll have to try some unconventional methods to achieve that. In my perfect little world, the brake lever would be rock solid, never moving even a smidge but only responding to the pressure of my fingers. Ahhh, to dream.
 
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