Bleeding brakes: Impossible due to system design??

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Hi,

Who agrees on this statement:

1. When bleeding brakes the main reason is to remove water/moisture from the hydraulic fluid.
2. The main entrance of moisture/water is at the rubber seals or o-rings.
3. When bleeding the calipers there is a large fluid compartment where the piston is located (back side of piston). The more the brakes are worn the bigger this volume becomes. But the entrance of the fluid and the exit (bleed screw) only make up a very small area/volume of the entire compartment. When bleeding the brakes the fluid passes more or less directly from the entrance to the exit and ONLY VERY LITTLE mixing happens. When bleeding the fluid does not travel across this big compartment but the shortest distance close to the outer zone of said compartment. Almost NO fluid is drawn from the area furthest away from this entrance/exit zone. This is pure physics/fluid dynamics. Due to this natural phenomenon of fluid dynamics (that the fluid travels the path with the least resistance), the fluid in the caliper is hardly changed whereas the fluid in the brake lines is changed 100% for sure. For this reason the water/moisture builds up over time even though the user believes everything is OK and the car is serviced correctly. Therefore rust and corrosion will form on the piston/cylinder walls even though the system was serviced hysterically by bleeding every year. Over time the caliper has to be replaced entirely.

In order to prevent this problem during bleeding, it is necessary to remove the entire caliper and squeeze the piston entirely back. By doing this, the fluid in this compartment is reduced to a minimum. Then one should let the piston move outwards again thus filling the compartment with fresh fluid (something that NEVER happens during a normal bleed process). Once it is out, it can be squeezed back a few more times in a similar fashion in order to reduce the amount of contaminated fluid in the compartment. All this of course requires much more work during bleeding.

The same applies for the brake cylinders with drum brakes: Due to system design the fluid closest to the rubber seals is hardly changed and just increases in moisture content over time.
 
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You are overthinking this. The procedure for bleeding the brakes is meant to remove any air from the system, since that would be the biggest factor contributing to poor braking performance. Removing moisture is done through exchanging as much of the old fluid as possible. I'm sure it isn't possible to get all of it out.
 
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I always thought that the problem with moisture in the the hygroscopic brake fluid was the boiling at high temp, causing air bubbles in the lines and loss of brakes at the bottom of that very long steep hill? I would want the moisture out!
 
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It's more that brake pads lose their friction and stop working when hot. Consider that big trucks, which don't have brake fluid, also experience brake fade from descending mountains improperly.
 
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But look at what DOES get changed out:
Reservoir
Master cylinder
lines to ABS pump
ABS pump (if cycled)
Lines to each corner of the vehicle, including distribution valves
SOME of the fluid in the caliper, along with any air or moisture, the amount of which is dependent on the placement of inlet and bleed screw.

So, maybe 80% or more of the system (?) The new fuid MAY absorb any leftover moisture in the calipers, reducing further corrosion or tendency to boil.
So, I wouldn't stop doing fluid maintenance because a little bit of old stuff might be left in a caliper.
 
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It's more that brake pads lose their friction and stop working when hot. Consider that big trucks, which don't have brake fluid, also experience brake fade from descending mountains improperly.
Yes, but you really cannot compare the two due to different demands, loads etc.

Pad fade on regular vehicles is really rear, especially with ceramic pads (one of the main reasons why manufacturers use them, although they have worse performance if we look at single braking). However, neglected fluid can lead to fading. The reason is that your average ceramic pad on let's say Toyota Highlander can sustain pretty high temperatures as the manufacturer already knows that the average buyer of Highlanders has a love affair with the brake pedal. But that heat will transfer to brake fluid, which is not a problem as long as the moisture level is OK.

Pad fade on track does happen a lot bcs. people who track cars usually do their homework regarding brake fluid. But gauging the right pad for a track or vehicle is a bit more complicated, and can be prohibitively expensive, especially on appliance vehicles used on the track. That is why on track, the most common conversation does not include oil, tires, or hp, but the selection of pads.
 
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I ntoice my vehicles tend to get a softer and softer pedal as time goes by, regardless of bleeding. Maybe this is why.
 
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The main entrance of moisture/water is at the rubber seals or o-rings.

I was thinking that it was getting in through the cap, which is vented. If I'm right about this, changing out fluid in the reservoir might be more important than bleeding, since this can be done in between brake bleeding.
 
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This subject where the moisture gets through is easily researched and has been discussed like 10,478 times here:


"Brake fluid can absorb moisture in a variety of ways – through the packaging process, while pouring it into the reservoir and even through the brake fluid lines."
 
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You are overthinking this. The procedure for bleeding the brakes is meant to remove any air from the system, since that would be the biggest factor contributing to poor braking performance. Removing moisture is done through exchanging as much of the old fluid as possible. I'm sure it isn't possible to get all of it out.
Yep. This is why before bleeding, we remove the old fluid from the reservoir (suck it out with a turkey baster or whatever) and replace it with new. Then as you bleed each brake, you draw fresh fluid from the reservoir through the lines to the caliper. Those lines have considerable volume, as you will notice by (A) how many pedal pushes it takes for clean fluid to come out at the caliper (you are repeating the pedal pushes until fresh clean fluid comes out, right?), and (B) how after doing each wheel, the reservoir level drops and you must top it off before doing the next wheel.

Sure, it leaves some residual old brake fluid in the system. No big deal. Just like when you change your oil it leaves some residual old oil in the engine.
I ntoice my vehicles tend to get a softer and softer pedal as time goes by, regardless of bleeding. Maybe this is why.
A thorough bleeding should restore the pedal like new. If the pedal gets soft, you may have a less than ideal bleeding procedure that leaves too much old fluid behind, a leak in the system, etc. A leak may not have external evidence; it could be fluid slipping past a bad seal in the master cylinder, back into the reservoir.
 
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Hi,

Who agrees on this statement:

1. When bleeding brakes the main reason is to remove water/moisture from the hydraulic fluid.
2. The main entrance of moisture/water is at the rubber seals or o-rings.
3. When bleeding the calipers there is a large fluid compartment where the piston is located (back side of piston). The more the brakes are worn the bigger this volume becomes. But the entrance of the fluid and the exit (bleed screw) only make up a very small area/volume of the entire compartment. When bleeding the brakes the fluid passes more or less directly from the entrance to the exit and ONLY VERY LITTLE mixing happens. When bleeding the fluid does not travel across this big compartment but the shortest distance close to the outer zone of said compartment. Almost NO fluid is drawn from the area furthest away from this entrance/exit zone. This is pure physics/fluid dynamics. Due to this natural phenomenon of fluid dynamics (that the fluid travels the path with the least resistance), the fluid in the caliper is hardly changed whereas the fluid in the brake lines is changed 100% for sure. For this reason the water/moisture builds up over time even though the user believes everything is OK and the car is serviced correctly. Therefore rust and corrosion will form on the piston/cylinder walls even though the system was serviced hysterically by bleeding every year. Over time the caliper has to be replaced entirely.

In order to prevent this problem during bleeding, it is necessary to remove the entire caliper and squeeze the piston entirely back. By doing this, the fluid in this compartment is reduced to a minimum. Then one should let the piston move outwards again thus filling the compartment with fresh fluid (something that NEVER happens during a normal bleed process). Once it is out, it can be squeezed back a few more times in a similar fashion in order to reduce the amount of contaminated fluid in the compartment. All this of course requires much more work during bleeding.

The same applies for the brake cylinders with drum brakes: Due to system design the fluid closest to the rubber seals is hardly changed and just increases in moisture content over time.
Where did you read this load of misinformation? I agree with nothing, IMHO it is just a bunch of hogwash. Brake fluid is hygroscopic, look it up.
Keep this in mind, theory and reality are two completely different kettles of fish.
 
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Hygroscopic brake fluids were chosen not despite but because of their hygroscopic properties. Moisture invariably finds its way into the brake system past seals and gets, thanks to hygroscopicity, distributed evenly in the brake fluid throughout the system. If brake fluid were not hygroscopic higher water concentration in some areas of the hydraulic system would cause the water to more easily boil and become gaseous, affecting stopping power dramatically. Corrosion would also increase in those areas. The more hygroscopic a brake fluid is, the more frequently it should be replaced. The system is flushed from the front to the rear. The new fluid goes in until the old fluid has exited the system and runs out clean and without air bubbles (tap the caliper with a mallet while bleeding)., Speed bleeders make this really convenient because they have a one-way valve and won't let air get back into the caliper. If you have to replace a caliper, or after a rebuild, I would fill the caliper with brake fluid before installing it, then flush and bleed.
 
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Hygroscopic brake fluids were chosen not despite but because of their hygroscopic properties. Moisture invariably finds its way into the brake system past seals and gets, thanks to hygroscopicity, distributed evenly in the brake fluid throughout the system. If brake fluid were not hygroscopic higher water concentration in some areas of the hydraulic system would cause the water to more easily boil and become gaseous, affecting stopping power dramatically. Corrosion would also increase in those areas. The more hygroscopic a brake fluid is, the more frequently it should be replaced. The system is flushed from the front to the rear. The new fluid goes in until the old fluid has exited the system and runs out clean and without air bubbles (tap the caliper with a mallet while bleeding)., Speed bleeders make this really convenient because they have a one-way valve and won't let air get back into the caliper. If you have to replace a caliper, or after a rebuild, I would fill the caliper with brake fluid before installing it, then flush and bleed.
Problem with hydrophobic brake fluids like Castrol SRF, which every new owner of M2/3 , Corvette or GT-R etc. wants on track bcs. racing fluid. When you tell them they have to flush it 3-4ltrs through the system to get rid of any bit of hygroscopic fluid, they complain it is $68 a bottle. Then bcs. the price they do not change in time and end up with gas pockets.
 
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Hi,

Who agrees on this statement:

1. When bleeding brakes the main reason is to remove water/moisture from the hydraulic fluid.

Disagree. Bleeding is done to remove air. Flushing is done to remove old fluid and to replace it with new fluid. First, you flush, then you bleed.

2. The main entrance of moisture/water is at the rubber seals or o-rings.

I'd say atmospheric contact with brake fluid is unavoidable and can only be minimized but not prevented altogether. the moisture concentration in the brake fluid will increase over time. Hence the service interval indicated in the service manual.

3. When bleeding the calipers there is a large fluid compartment where the piston is located (back side of piston). The more the brakes are worn the bigger this volume becomes. But the entrance of the fluid and the exit (bleed screw) only make up a very small area/volume of the entire compartment. When bleeding the brakes the fluid passes more or less directly from the entrance to the exit and ONLY VERY LITTLE mixing happens. When bleeding the fluid does not travel across this big compartment but the shortest distance close to the outer zone of said compartment. Almost NO fluid is drawn from the area furthest away from this entrance/exit zone. This is pure physics/fluid dynamics. Due to this natural phenomenon of fluid dynamics (that the fluid travels the path with the least resistance), the fluid in the caliper is hardly changed whereas the fluid in the brake lines is changed 100% for sure. For this reason the water/moisture builds up over time even though the user believes everything is OK and the car is serviced correctly. Therefore rust and corrosion will form on the piston/cylinder walls even though the system was serviced hysterically by bleeding every year. Over time the caliper has to be replaced entirely.
Practically speaking, if you flush the brake system until you see clean brake fluid exit the system you have pretty much flushed the caliper cylinder. Will there be some sediment and wear particles remaining? Yes but the amount will be trivial and negligible. If you want perfection, remove the caliper, rebuild it, and fill it with fresh brake fluid before installing it. You then should probably also remove and rebuild and refill the ABS control module because it too gets probably imperfectly flushed even if you cycle it with the appropriate computerized tool. Perfection is the enemy of perfectly adequate!

In order to prevent this problem during bleeding, it is necessary to remove the entire caliper and squeeze the piston entirely back. By doing this, the fluid in this compartment is reduced to a minimum. Then one should let the piston move outwards again thus filling the compartment with fresh fluid (something that NEVER happens during a normal bleed process). Once it is out, it can be squeezed back a few more times in a similar fashion in order to reduce the amount of contaminated fluid in the compartment. All this of course requires much more work during bleeding.

Unless you rebuild the caliper and install new piston seals and either replace the brake pistons or ensure they are perfectly smooth the process of running the brake piston throughout its full range is not without danger. You put tens of thousands of miles of equivalent wear on the brake piston seals.

The same applies for the brake cylinders with drum brakes: Due to system design the fluid closest to the rubber seals is hardly changed and just increases in moisture content over time.

Hygroscopic brake fluid has the same water concentration throughout its whole volume.
 
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1) Air in a hydraulic brake system will compress when pressure is present to cause brakes to come on, and this will prevent the normal amount of high pressure from being applied to the wheel cylinder(s) and thus prevent the brakes from working properly. So, of course there can not be air in the hydraulic system.

2) When a hydraulic system gets hot enough IF water is present in significant amounts that the brake fluid can no longer contain it, then that water will turn into steam which is a gas. Steam will expand when the brake is released and brake fluid flows back to the master cylinder(s). That expanded gas is compressible, and the next time the brake is applied it will compress and high pressure will not reach the wheel(s) to cause the brake(s) to work properly.

3) ABS systems in general have an aluminum section containing machined passages. And, brake lines are made of a mix of several metals to make an alloy that has the desired properties such as being strong enough to contain the high pressure and soft enough to be formed into bends without breaking, and soft enough to not be brittle when subjected to jolts and vibration when the vehicle is in use. And one of the metals used to give brake lines this property is copper. Copper and Aluminum are on opposite ends of the electronegativity chart. Which means they can be used to make a good battery, but also means that when these two metals are in proximity to each other and enough moisture is present to allow electrons to flow, corrosion will happen. Over time some of the copper in the brake lines does get into the brake fluid, and since the brake fluid moves, some of it will find its way into the brake fluid that is inside the ABS system. Add to this enough water content to allow electrons to flow and this is a very bad thing to have happen in the aluminum passageways of an ABS system. This will render the ABS system unusable. So, this is by it self would be a significant enough reason to flush brake fluid to get out all fluid when the water content (and copper content) is increasing, and before it becomes high enough to cause this problem.

Also, water content in brake fluid can cause corrosion in the inside of brake lines, and the cylinder and or piston of wheel cylinders and master cylinders.

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Bosch ES16 is a very good brake fluid that prevents water from causing problems when that water concentration is a low levels that standard DOT 3. and DOT 4, and DOT 5.1 would have problems with. That said, it still has limits, though higher. So, it probably can be ran for a longer time before requiring being changed. It cost a lot more than standard brake fluid, so there is that to consider when deciding if you want to use it. I use it and I bought 2 of the one quart containers from Amazon when I flushed the brakes on my 2016 Honda CR-V EX AWD a little over a year ago. BTW, the front with the larger calipers and larger pistons uses a lot more fluid to flush it out than the back does even with the longer lines to the back.

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Also, as per the post, if you compress the wheel cylinders then you will not be leaving as much in the wheel cylinder. And always open the bleeder before compressing so you do not force dirty wheel cylinder fluid back up into the system where it might find its way all the way to the ABS system and cause problems. Sometimes wheel cylinder fluid does have a lot more dirt it it. I had one on the passengers front that came out all black and the others were nowhere near as dirty.
 
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I’ve been in the automotive fleet industry for 42 years. All of that time with first responder and law enforcement equipment. In my experience moisture and air enters the system via the master cylinder. If it could enter any other way there would be a breach/leak in the overall braking hydraulic system. The recreational brake flush madness for street driven passenger vehicles is designed for one thing - take your money. Leave the master cylinder top on and only remove when adding fluid. Our fleet was forced to run 200k due to budget reasons. No brake failures due to not flushing brake fluid, mysterious air, and mysterious moisture entry in the system. There was always a mechanical reason for any moisture and/or air entry. I’m old.. I wish you well with your brake fluid flushes. Rounded off bleeders ain’t my idea of fun. That’s my opinion.
 
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down in the park
It's more that brake pads lose their friction and stop working when hot. Consider that big trucks, which don't have brake fluid, also experience brake fade from descending mountains improperly.

That phenomenom causes what appears to be hard pedal, as if jammed (it's not, but the brakes are ineffective no matter how hard you press). Boiling brake fluid causes a soft pedal. You have limited brakes in either case though
 
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there is a large fluid compartment where the piston is located (back side of piston). The more the brakes are worn the bigger this volume becomes. But the entrance of the fluid and the exit (bleed screw) only make up a very small area/volume of the entire compartment. When bleeding the brakes the fluid passes more or less directly from the entrance to the exit and ONLY VERY LITTLE mixing happens.
I'm not sure that this "compartment" is as large as you think or holds as much fluid as you think. Maybe 250ml of fluid in a typical caliper ? Plus, isn't the entire volume of the brake lines, caliper, etc 100% full so any "pushing" is moving ALL of the fluid in each line ?
 
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