Brake Fluid Question

joesixpack

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Brake Fluid and You,

While troubleshooting a problem with my brakes the other day, I began to wonder about brake fluid and what makes it so special. In almost all cars, the required fluid is "DOT-3" hydraulic fluid. For those of you who've never had the pleasure of dealing with it, it has the look and consistancy of baby oil and the alarming ability to ruin any paint it comes in contact with (no matter how brief the encounter).

In order for a fluid to be DOT-3, it has to fulfill at least these three requirements (there may be more, I just havn't figured them out yet);

1) It cannot cause rubber and plastic seals to deteriorate. Seems a pretty obvious requirement, as all the seals in the brake system fit this description.

2) It is Hygrostatic (it's always fun to learn new words!). That means it's the same specific gravity as water, so that if water gets into the brake lines, it will not seperate out the way it will with oil. This seems to be very important.

3) It has a very high boiling point (generally over 500 F.). There are two boiling points listed on the fluid's data sheet. The regular boiling point and the "saturated" boiling point. The saturated boiling point is the boiling point for the fluid with a 3% concentration of water mixed in. This is a lower number and one of the reasons race cars replace their brake fluid for every race.

Now, here's my question. Is the heat generated by braking being conducted through the brake shoes and pads, the pistons and cylinders, and then into the brake fluid? Or is there heat created in the fluid adiabaticly (spell check, please) by compressing the dissolved gas in the fluid?

I personally think it's the first case, as the heat generated adiabaticly would dissipate as soon as you took your foot off the brake pedal, however my co-worker disagrees. Is there a physicist out there who can answer my question? (Shade-tree mechanics need not answer, unless they also have some physics education.)
 
joesixpack said:
Now, here's my question. Is the heat generated by braking being conducted through the brake shoes and pads, the pistons and cylinders, and then into the brake fluid? Or is there heat created in the fluid adiabaticly (spell check, please) by compressing the dissolved gas in the fluid?
Click to expand...

There is negligible volume change under pressure for the fluid, so there is no significant heat from compression in the first place. The relevant heat is indeed the heat being conducted from the brake pads.

The reason this is an issue, of course, is that if you get it hot enough that vapor forms in the line, then that vapor has a huge compressibility (that is, large volume change when the pressure changes). Brake systems are designed to apply a large force over a small distance, which works when the fluid is fairly incompressible, but fails when the fluid becomes compressible. So you have a bubble in your brake fluid line, and when you try stepping on your brakes again (or if they ease up and try to reapply pressure during the pulsing of an antilock brake), instead of pushing down hard on the brake pad, you just get that bubble of vapor in the line shrinking with only a little force applied to the brake pad. And so you lose braking power, with likely dire consequences for the car and its occupants. This is also why #2 is important: if it isn't hygrostatic, an opportunity exists for water to separate out from the brake fluid near the brake pads. And since water has got a low enough boiling point, that's pretty much a guaranteed recipe for forming vapor bubbles in the brake line. But if water can disolve in the fluid, then you shouldn't ever form such pockets of water.
 
The heat is generated by the shoes against the disk/drum. Usually, this heat is radiated directly into the surrounding air. Many disk brakes have hollow disks to facilitate this. Motorcycle disks usually have holes in them for extra heat dissipation.

Under heavy brake use, the pads can conduct heat into the piston and into the brake fluid. Obviously, the brake fluid should not degrade under these conditions. This is probably the reason for requirement 3). But this is not the normal route of heat loss.

There should not be any gas in the fluid, dissolved or otherwise. This would allow a greater travel of the master cylinder for a given pressure; the pedal would have a "spongy" feel and would not reach full effectiveness before the pedal hit the floor.
 
You are right, without a doubt.

Unless there's air in the lines, brake fluid is not compressible in the first place. Even if it were, how much power can your leg generate, compared to how much your brakes can dissipate? Perhaps a horsepower or two vs. hundreds or thousands.

If you drive around with your parking brake on, you'll overheat your brakes. Parking brakes are cable-operated; no compression at all of brake fluid occurs.

Challenge your coworker as follows: You agree to touch the brake lines at the wheel after he heats them up by pressing the brake pedal with the car motionless, if he agrees to touch them after you heat them up by braking from high speed using the parking brake. :D
 
joesixpack said:
In order for a fluid to be DOT-3, it has to fulfill at least these three requirements (there may be more, I just havn't figured them out yet);
Click to expand...
Knock yourself out: Part 571 - federal motor vehicle safety standards.

Try "Standard No. 116; Motor vehicle brake fluids".


joesixpack said:
It is Hygrostatic (it's always fun to learn new words!). That means it's the same specific gravity as water, so that if water gets into the brake lines, it will not seperate out the way it will with oil.
Click to expand...

I'm curious - where did you happen to get that word (and that definition of it)? It would be a new one for me, too.

I've never heard of brake fluid having to be the same density as water. I have heard of it being hygroscopic (absorbs water), which indeed, DOT 3 and DOT 4 both are.
 
Re: Re: Brake Fluid Question

_Q_ said:
...I've never heard of brake fluid having to be the same density as water. I have heard of it being hygroscopic (absorbs water), which indeed, DOT 3 and DOT 4 both are.
Click to expand...

I've always noticed that it's runny like water. Is there some viscosity requirement, or is this a natural consequence of it being hygroscopic/hygrostatic/whatever?
 
Re: Re: Re: Brake Fluid Question

phildonnia said:
I've always noticed that it's runny like water. Is there some viscosity requirement, or is this a natural consequence of it being hygroscopic/hygrostatic/whatever?
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Yes, there's a viscosity requirement.

S5.1.3. Kinematic viscosities. When brake fluid is tested according
to S6.3, the kinematic viscosities in square millimeters per second at
stated temperatures shall be neither less than 1.5 mm\2\/s at 100
deg.C. (212 deg.F.) nor more than the following maximum value for the grade indicated:
Click to expand...
 
Re: Re: Re: Brake Fluid Question

phildonnia said:
I've always noticed that it's runny like water. Is there some viscosity requirement, or is this a natural consequence of it being hygroscopic/hygrostatic/whatever?
Click to expand...
According to the link I included in the earlier post, minimum (at 100 degrees C) and maximum (at -40 degrees C) viscosities are specified.
 
I'm curious - where did you happen to get that word (and that definition of it)? It would be a new one for me, too.
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Do'h! Sorry, you're right, the word IS "hygroscopic" and I guess I defined it improperly. I had read that it (brake fluid, i.e.) would mix readily with water and took that to mean that they were both the same density. I guess that's not strictly true, though. Fluids with different densities can still mix.
 
Don't want any chance of freezing,(Or expansion/ contraction with temperature variation), or corrosion.
Is it a synthetic oil, or perhaps a silicone?

You just take stuff like this for granted, till someone asks.
 
Thanks for all the replies. I guess the more I think about this the more apparent it becomes. Obviously there is no compression of the fluid, and any gas disolved into the fluid is negligable. And clearly, the brake lines won't heat up if you're parked and pumping the brakes all day long. 69dodge had a good idea, and I should have my friend take that bet.
 
Soapy Sam said:
Is it a synthetic oil, or perhaps a silicone?

You just take stuff like this for granted, till someone asks.
Click to expand...
Unless you happen to read car-related sites where stuff like this gets brought up (and beaten to death) with some regularity! Ed forbid that you should ever ask there which sort of motor oil to use, as a riot might ensue.

From what I've read, DOT 3, 4, and 5.1 are glycol-based products (I see the term "polyalkylene glycol ether" used some).

DOT 5 is silicon-based, a different animal (not hygroscopic, more compressible) and not to be casually substituted for the other fluids!

Again, only what I've read - your Web search engine of choice will turn up all that and more.
 
Ed forbid that you should ever ask there which sort of motor oil to use, as a riot might ensue.
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LOL, that's a truly funny debate to listen to (or read). It's amazing how staunchly people defend their motor oil choice and denegrate other's. It's frikkin' MOTOR OIL, how different can one brand be over another?

Me, I prefer what's on sale.

The other funny motor-head debate is fuel octane. I can't believe how many people buy high octane gas for a low-compression engine, thinking they'll get more power, better mileage, or cleaner fuel injectors.
 
I've heard that milk is perfectly acceptable replacement for brake fluid. Well.. at least for old soviet cars.
 
Just a shade tree mechanic here with very little in the way of formal physics training.

However, I'd be willing to bet that the vast majority of heat absorbed by brake fluid comes from the fact that master cylinders are located under the hood, next the engine. Plus, if you've ever worked on brake lines you'd know, that they typically run down to the frame right past the exhaust manifold. This would explain the 500 degree boiling point spec..
 
I think that it is possible that the majority of the total heat absorbed by the brake fluid comes from the engine compartment as you said.

But this doesn't sound like the critical issue as far as the requirement for a high boiling point. Brake parts get really hot and the brake fluid is going to be exposed to high temperatures as a result and I suspect it would be a bad thing to have the brake fluid boiling away while one is trying to stop one's car going down a mountain.
 
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