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  #1  
Old 02-11-2003, 02:05 PM
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Disappearing "phantom" Brake Fluid

I have encountered a very unusual event on my 97 e320. The car has new OEM rotors, brake pads, and a fluid change done by the dealer. I checked this morning in the garage and the fluid was below min. Then I drove to the mechanic and it was at maximum. Please advise.
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Old 02-11-2003, 03:36 PM
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All fluids, including brake fluid, expand when heated.
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Old 02-11-2003, 04:26 PM
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I don't think it is expansion because in So. Cal the temp does not fluctuate that radically and it was in a garage.
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Old 02-11-2003, 04:57 PM
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Did you park on a level surface?

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  #5  
Old 02-11-2003, 08:20 PM
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Yes, the car was on a level surface. While in the garage, I even turned on the engine and inched back and forth but no change in brake fluid level. But after driving 7 mi to mechanic, it was at max.
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  #6  
Old 02-11-2003, 11:17 PM
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Not ambient temperature!

Regardless of the temperature fluctuation in California or a garage, the temperature of your engine bay fluctuates ~ 150 degrees. When brake fluid gets hot, either from the brake itself or otherwise, it expands a great deal - hence air brakes on trucks. Why don't you check it as the car cools and see if there is a correlation? Maybe one of your new components is dragging and heating up one of the brakes. Have you checked each wheel for heat after driving? How did the rotors get to the point they required replacing in the first place?
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  #7  
Old 02-12-2003, 11:49 AM
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For all practical purposes, liquids do NOT expand, and this is a determining factor in the use of hydraulic brakes instead of mechanical. What causes problems is the introduction of gases in the brake system, such as from the change of state of the brake fluid (ultra-low vapor pressure - miniscule contribution), but mostly from contaminants - such as absorbed water. Air in the lines is another contributor. It is safe to say the cause of what is happening to royaiii is NOT expanding brake fluid - look elsewhere.

Perhaps a ruptured seal in the power brake mechanism is sucking fluid out, trapping it, and allowing it to return when the engine is off. VERY bad brake fluid contamination might also be an issue, but I would expect dragging brakes or other accompanying symptoms.

Steve
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Old 02-12-2003, 01:57 PM
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sbourg:
I respectfully disagree and I think you're splitting hairs. Whereas expansion of the fluid may or may not be the problem, the fluid level would increase when hot. From a practical standpoint, moisture and air are components of brake fluid. PV=nRT or if you want to get really picky (P+n2a/V2) x (V-nb) = nRT - both proven beyond your opinion. Do you check your trans fluid cold or hot and why? Whether that expansion is technically due to pressure from gases produced, contaminants - whatever - the fluid still increases in volume. As a matter of fact, volume is often a measure of (invisible) gases displacing (visible) fluids. Whether royaii may actually be witnessing expansion of the actual pure fluid or not - the volume will still increase as seen in the master cylinder - which is the situation he is seeking an explanation for. I didn't think the specific physics were required in my posts as this is MercedesShop and NOT PhysicsShop. "Very bad brake fluid contamination" is unlikely as the initial post stated that the fluid was just changed. If he has a dragging brake, it will heat that caliper enough to show the increase in fluid in the master cylinder as the inherent gases expand and increase the volume. I agree with you that this is the most likely explanation, however if you do NOT believe the fluid level will increase as a result of heat I do NOT know how you can support your own seemingly contradictory conclusion.

PS. While I welcome any opposing views, I do NOT appreciate the yelling. I'm just trying to help out here as I have received a lot of help here am actively trying to give back.
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Old 02-12-2003, 04:35 PM
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"however if you do NOT believe the fluid level will increase as a result of heat I do NOT know how you can support your own seemingly contradictory conclusion. "

I do not believe the fluid level will change noticeably as a result of heat in a properly charged brake system. It never has for me. On the other hand, air is the most common brake system contaminant, and it will behave according to the formula your gave for a gas. If the system was improperly bled when the system was purged, that could explain the problem, as I said. Likewise if all the water was not purged - the brake fluid can exceed 100C, at which point the water will become entirely a gas.

You may have known what you meant, but both times you posted, you stated that the brake fluid will expand with heat. It will not. You are free to disagree, but that will not help the original poster resolve his problem. If you find any contradiction in what I posted, you have not indicated where.

Now where did anyone shout?

Steve
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Old 02-12-2003, 07:01 PM
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sbourg:
The shouting in your post is indicated by the capitalized "NOT"'s which I tried to point out by duplicating at the end of my own post. Maybe you are not aware that capitalization = shouting. If you are not, I apologize for taking offense. The formula's I gave are not only for gas, they are also for liquids as a high enough T turns them into gas. "Liquid" and "gas" only refer to the state something is in at atmospheric pressure and a specific temp and can be manipulated by those two same variables. Brake fluid is no majic liquid. It behaves the same way. Higher temps will drive some of into a gas state, regardless of the amount of air. Air in the system would actually give the volume of fluid somewhere to go as air will compress and then you would be even less likely to see a change at the master cylinder. Same reason brakes don't work when they have a lot of air in them - it compresses and no work is done. At any rate, at a high enough temp (a badly dragging brake can create temps in the neighborhood of 300 C) the brake fluid itself becomes a gas (reaches it's boiling point). DOT ratings on brake fluid have a lot to do with boiling points as a result. I have seen a dragging brake blow the cap of off a master cylinder. My objection was (is) based on my interpretation of your yelling as condescension and that with regards to my word choice, splitting hairs. Here's an analogy to explain what I mean: Someone asks about why to leave a nail's width gap between boards on an outside deck. I answer because wood expands and contracts with change in temperature. You then counter with an emphatic answer stating that for all practical puposes, wood does NOT expand, but that the air trapped inside and between the individual fibers and cells may, but that is safe to say the wood, per se, expanding is not the reason. See, in reality, for all 'practical' purposes, wood and liquids do expand. It is only 'technically' that they do not.
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Old 02-13-2003, 11:12 AM
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bjcsc, to apply the gas law to a system comprised of a liquid and gas at equilibrium is just incorrect. Also, brake fluid is formulated with a very high boiling point, very low viscosity, and very low vapor pressure. It is specifically designed to NOT do exactly what you claim, and outside of steep mountain descents or road racing, any shift in level at the master cylinder should be unrelated to this.

You claim the phenomenon the original poster reported is possibly caused by expanding brake fluid in a normally functioning brake system. If you still want to hold by that opinion, you are welcome to it. I say that is not the case.

Caps lock on is shouting, occasional capitalization is emphasis only in text messaging. But, I can't alter how you elect to interpret it.

Steve
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Old 02-13-2003, 06:37 PM
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sbourg:
This will be my last post on this subject and I will yield the last word to you if you so choose, for you can lead a horse to water but... I want to make a few things clear: 1) My initial post to royaiii was not to suggest that his engine warming up was causing his problem, but I was trying to point him in the direction of heat so he may find a dragging brake himself. (teach a man to fish vs. give a man a fish) When he didn't, I gave it to him in the followup post. 2) Your quote " You claim the phenomenon the original poster reported is possibly caused by expanding brake fluid in a normally functioning brake system" is inaccurate. My assertion the whole time is that he probably has a dragging brake, which is common after new pads are installed. 3) Your quote "bjcsc, to apply the gas law to a system comprised of a liquid and gas at equilibrium is just incorrect" could not be more in error. The second equation I gave you (after stating 'if you really want to be picky - and you do) was Van der Waals' equation. It does just that and he won the Nobel Prize for the development almost a century ago. See the excerpt below and the link for a graphic. If you could prove your statement, you might very well be in contention for your own Nobel prize as it would change the face of modern physics. 4) Your statement "It is specifically designed to NOT do exactly what you claim, and outside of steep mountain descents or road racing, any shift in level at the master cylinder should be unrelated to this", I agree with. It is precisely why I asked him how his rotors were destroyed in the first place (could be his driving or his conditions, he may very well live on some mountaintop somewhere) and finally 5) Your quote "Caps lock on is shouting, occasional capitalization is emphasis only in text messaging. But, I can't alter how you elect to interpret it" If this is the case, what is the purpose of bold text? Caps = shouting on any of the many forums I visit and bold = emphasis. I don't interpret it this way, it's often in the forum information pages. I believe your intention was clear especially when your use of caps is viewed in the context of your post.


VAN DER WAALS, JOHANNES DIDERIK (1837 - 1923). Over de Continuiteit van den Gas- en Vloeistoftoestand. Leiden, 1873.

In 1873, van der Waals obtained his doctor's degree for a thesis entitled Over de Continuiteit van den Gas- en Vloeistoftoestand (on the continuity of the gas and liquid state), which put him at once in the foremost rank of physicists. In this thesis, he put forward an "Equation of State" embracing both the gaseous and liquid state; he could demonstrate that these two states of aggregation not only merge into each other in a continuous manner, but that they are in fact of the same nature. This equation of state was a dramatic improvement over the ideal gas law. It was van der Waals' genius that made him see the necessity of taking into account the volumes of molecules and the intermolecular forces ("van der Waals forces", as they are now generally called) in establishing the relationship between the pressure, volume, and temperature of gases and liquids. This copy of Over de Continuiteit is open to the chapter where van der Waal's equation of state, ( P + a/V2 )( V - b ) = RT, is first derived. Van der Waals won the Nobel Prize for this work in 1910.

Partington IV, pp638-640; Nobel Lectures, Physics, 1901-1921.

Van de Waals' Equation Graphic

bjcsc
B.S. Molecular Biology 1992
B.S. Physics 1992
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Old 02-13-2003, 08:01 PM
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bjcsc, since you went to all the trouble to pull up a website on Van der Waal's equation, plug in the values for a liquid. Pressure is certainly strongly related to temperature - but volume is virtually unaffected. Then reread your posts - especially the first - and you'll understand my objection.

Increased verbosity doesn't help the poster - and misinformation or misconstruing whatever it is you are claiming won't help either.

Steve
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  #14  
Old 02-13-2003, 08:41 PM
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although i found your ....

diatribes interesting at first , i quickly lost interest.i'm sure you both meant well but intentions aren't always realized.maybe you two could go here to finish your 'conversation':
www.i'll argue anything 'til mom kicks me out of the basement
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