dumb elementary brake fluid question

[berninicaco3]

So, I was trying to explain to a friend the reasons why he should change his brake fluid at least every 15 years, and realized that I myself have a hole in my understanding of hydraulics.


A reason I was given was that as the fluid absorbs water, the boiling point is lowered. When the fluid boils, it becomes a gas (of course), and gasses are compressible. Compressible gasses = spongy pedal.
From wikipedia, mirroring my memory almost word for word:

"Fade can also be caused by the brake fluid boiling, with attendant release of compressible gases. In this type of fade, the brake pedal feels "spongy". This condition is worsened when there are contaminants in the fluid, such as water, which most types of brake fluids are prone to absorbing to varying degrees. For this reason brake fluid replacement is standard maintenance."


So here's where I'm at a loss.
You have a fixed volume. Unless your master cylinder has a bad seal, or your caliper(s), it's an enclosed system. If it gets hotter, where is there ROOM for a gas pocket to form?
We were taught in grade school that the very definition of a liquid/fluid, is that it is non compressible.
For a gas to make space for itself in a closed system full of fluid that's been heated, either there's a leak to the outside volume, or, fluids ARE compressible but just to a lesser extent than gasses? The latter seems more likely.
But even if you filled a pressure cooker with brake fluid and no air gap and turned on the heat, and the fluid gets denser while air bubbles form in the same volume... the air bubbles are still under the exact same pressure, and should still transmit the same forces, right?

So the pedal cannot fade to the floor. If the air bubbles are from boiled fluid in the same volume, they should be under the same pressure and transmit your braking force just as surely as the fluid would have, no?


I may be missing something fundamental in my education about fluid dynamics (admittedly not that much education), and I'd like to understand more thoroughly.

 

[berninicaco3]

second hydraulics question.

Elementary idea is fixed volume, push in here, comes out there; you can force multiply easily enough by pushing a small rod a long distance and having a wide plate rise a short distance (but with more force).

In between, however, does not matter, correct? Not... in hydraulics 101.
Whether your brake lines are 1/4", 3/16", or even 1/32" capillaries or 2" pipes, does not factor into the simplistic explanation.

In reality there is some friction with the inner lining of the brake lines, so the larger they are, probably the faster you can transmit force (more volume to surface area, so less sticking to the inner walls).
Maybe there's another factor that I'm overlooking.

This is silly, but have you ever gone to panera, or burger king, and had a frappe, and they give you this giant tube straw? At least 3/8" diameter if not 1/2". I actually find it difficult to draw the necessary suction.

Now, if you pinch one limited section of the pipe, does that affect much of anything?

Again, the simplistic view looked at only the beginning and end points. In between didn't matter. So if the straw is 3/8" at the bottom and 3/8" at the top, necking it down to 1/8" in the middle has no effect (in the very basic view).

But in reality, what does happen? What are the equations that govern it? Are they similar to equations for electrical resistance in a wire, and energy is lost?

And what happens when a brake line is pinched? Or if you install larger diameter fluid lines?

 

[229]

Change it every 2-3 years. I had to replace an expensive master cylinder that failed prematurely because it had excessive grit in it from the breakdown of rubber hoses. Would have been more economical to change the fluid regularly.

 

[yonyon]

Originally Posted By: berninicaco3

So here's where I'm at a loss.
You have a fixed volume. Unless your master cylinder has a bad seal, or your caliper(s), it's an enclosed system. If it gets hotter, where is there ROOM for a gas pocket to form?


The excess volume goes into the reservoir. With the brake pedal fully released the fluid is free to flow in either direction between the cylinder and the reservoir. If this were not the case, the pedal would drop with lining wear.

 

[andyd]

If you take a used caliper or a wheel cylinder apart it will be pitted, This is from air and water in the brake fluid . The wheels are the lowest point in the system and that is where the air and water collect. It is also where the fluid will boil. Air eventually bleeds upward to the MC cylinder. The boiling water pits the cylinder and raises the chrome plating on the pucks. The MC acts as an expansion chamber so you do not have a fixed volume

 

[cchase]

The brake system is not closed. There is a reservoir.

 

[Smoky14]

Minor error: fluids are compressible liquids by theory are not.

Fluid= a substance which will assume the shape of the container in which it is placed, thus a gas is a fluid.

 

[eljefino]

Originally Posted By: cchase
The brake system is not closed. There is a reservoir.


yup, this, and the top of the reservoir has a cap that lets a little air in and out.

When you blow a brake line they say "pump the pedal" because when the pedal is all the way up a little slit is opened and new fluid dribbles in from the reservoir.

 

[Brad_C]

Originally Posted By: berninicaco3

In reality there is some friction with the inner lining of the brake lines, so the larger they are, probably the faster you can transmit force (more volume to surface area, so less sticking to the inner walls).
Maybe there's another factor that I'm overlooking.


There may be. Have you ever compressed water before? I have to a noticeable extent (Okay, I was using a pretty sensitive apparatus, but still). Knowing this is against everything they taught me at school I did some digging. There is *always* a proportion of air dissolved in water, and I was compressing the air molecules trapped in the water. I wonder how much dissolved gas exists in brake fluid?

As a counter to bigger brake lines transmit more force faster due to low friction, how about bigger brake lines hold more fluid and therefore more dissolved gas leading to a higher compressibility?

Just a thought.

 

[GMorg]

If gases form in your pressure cooker example, if you blocked the pressure release, the pot will first distort and then explode. DO NOT try at home!

Small lines allows removal of trapped air since even small bubble make contact with the full diameter of the line -- they don't get trapped as fluid moves through the line. Small lines also flex less due to pressure. Lastly, as already mentioned, small lines hold small volumes which hold less gas and is thus less compressible.

I don't have many high performance situations on my daily driver where I need to worry about water dropping the boiling point of brake fluid. However, I do have concerns about water in lines causing corrosion. Much of the pitting in brake systems is due to corrosion.

 

[berninicaco3]

I think my questions have been answered by and large.

Sounds like water is a concern mainly because of corrosion, not lowered boiling point.

Sounds like brake fade from boiled fluid isn't the main issue anyway (fade from the hot rotorad lowered friction more the point).

And that perhaps spongy pedals are due to other concerns-- or air bubbles that became introduced from the outside, not really generated by boiled fluid? (those same air bubbles ought to re-condense when it all cools down, anyway!)

Or perhaps air in the lines is from BURNT fluid, which is another thing entirely.

Also still curious about small constrictions/ pinch points, the straw question.

 

[berniedd]

The brake slave cylinder in each wheel is already full of brake fluid. Small diameter brake lines are OK because the brake slave cylinders need to receive only a very small amount of additional brake fluid volume under pressure to actuate. It works without issue even if you partially pinch a section of the brake line. However a fully clogged/pinched tube is another, totally different situation.

 

[Kiwi_ME]

Originally Posted By: berninicaco3
Whether your brake lines are 1/4", 3/16", or even 1/32" capillaries or 2" pipes, does not factor into the simplistic explanation.
It does, but having a thousand or so PSI of pressure at your disposal makes the pressure drop rather insignificant.

Regarding the open system, the level in the reservoir changes very slightly when the pedal is up and therefore a vented reservoir (most seem to be these days) has an opportunity to breath in and out humid air.

 

[JZiggy]

You need to look up a concept called a PVT curve (pressure volume temperature).

If you raise the temperature of your "fixed" volume, the incompressible fluid evolves a compressible gas component. The volume then is no longer fixed because the gas can compress. The initial condition on the system was hydrostatic pressure, not fixed volume in fact.

For an incompressible fluid, you can increase hydrostatic pressure without changing volume (flat curve). For compressible substances, increased pressure decreases volume.

The change from incompressible to compressible is temperature dependent and also dependent upon how much water is in solution with the glycol. The brake fluid boils at 450F, water at 212F. The less water, the better

 

[JZiggy]

Also, about your question relating to brake line diameters. There's another concept called hyrdostatic pressure. During the instant that you press the brake pedal, yes the fluid on the master side is under greater pressure than the slave side. But once the fluid stops flowing, hydrostatic pressure is established (it's all under the same "compressing" forces, and there is no pressure differential causing flow).

If your brake lines were thinner, it would take more force to get the brake pedal down but then you would have the same braking force once you got there.

My only concern of very small brake lines or kinks would be cavitation. Ever pinched a garden hose while it's flowing and heard it hiss? That's the fluid turning into vapor momentarily due to the Bernoulli effect (IIRC).

 

[Ken2]

Quote:
Sounds like water is a concern mainly because of corrosion, not lowered boiling point.

Sounds like brake fade from boiled fluid isn't the main issue anyway (fade from the hot rotorad lowered friction more the point).
WRONG AND DANGEROUS.

Don't overthink things, then assume you have an answer when you don't know all the factors.

When a brake system boils the pedal goes to the floor. The brakes do not stop the wheels. There is nothing else to know. This is exactly how a friend described it when he was towing a horse trailer down Mt. Adams. His pedal went to the floor. The brakes did not work. He got stopped by downshifting and using the mechanical parking brake. After a period of time his brakes then worked again normally. (They cooled.) After this he flushed his brake fluid and downshifted for long downgrades when running heavy. And this is exactly how a state patrol report described a fatal wreck where a minivan driving down a very steep hill overheated the brake fluid.

Excess heat from heavy braking changes the brake fluid so it must be flushed. Smoking brake linings are damaged and must be replaced. The pads or drums must be carefully inspected for heat damage and possibly replaced.

http://stoptech.com/technical-support/technical-white-papers/brake-fluid
When fresh, all brake fluids are virtually incompressible and the system works as well as its mechanical and hydraulic design allows. There can be, however, significant problems in the proper functioning of brake fluid. Overheated brake fluid can (and will) boil in the caliper. Boiling produces gas bubbles within any boiling fluid. Gas is compressible so boiling brake fluid leads to a “soft” brake pedal with long travel. In extreme cases overheated brake fluid necessitates “pumping the brake pedal” in order to get a pedal at all.

This leads to a discussion of boiling points. Brake fluids are classified by both “dry boiling point” and “wet boiling point.”

 

[andyd]

Old brake hoses can fail internally . A piece comes loose and acts as a check valve holding pressure. It was a 25 yr old that this happened on, when it happened to me. Once. Hoses expand and use up pressure. When I did a major brake R+R on the Rat, I replaced the front hoses. Because they looked terrible,even though they were only 18 yrs old.

 

[berninicaco3]

"WRONG AND DANGEROUS.

Don't overthink things, then assume you have an answer when you don't know all the factors."

That's exactly why I'm asking-- I didn't think I had the answer right! I was proposing an answer so it could pointed out if it was wrong (and why).
I do like to overthink things.



So the big point that I was missing is that while yes, fluids remain ideally incompressible, the gas isn't (necessarily) in big bubbles of air pockets at all but rather dissolved in the brake fluid, thus making that fluid compressible now?

I haven't done a lot of mountain driving. Almost none. I was driving down pikes peak and we noticed a burning smell, and it was the brakes getting very hot, so we pulled over. No darkening of the fluid, no bluing of the rotors, no warping of the rotors. But I think we were just lucky to have caught it in time.

Do you want to pump all along, from when you first start going down a long decline, or, is pumping only the last resort emergency measure after you've noticed the beginnings of brake fade (and you need to pull over immediately and let the brakes cool properly)?

Yeah, that happened to me too-- although not directly relevant to questions of brake fluid. My right front caliper locked up; the car shook like a tire was five pounds out of balance. It took me a couple starts and stops to figure out just what had happened, since it had never happened to me before.
At least, the symptoms were consistent with that check valve explanation. I didn't play around and just replaced the hose and caliper together. This was on a rather old ford.

 

[berninicaco3]

question about engine braking.

It's hard to find techs who really, really know about transmissions; and that definitely includes myself. I haven't pinned down the answer to this one yet.

For engine braking to work, the wheels have to want to be going faster than the engine, no?
So that the wheels are trying to drive engine compression (rather than engine combustion powering the wheels).

This works a LOT better if you actually have release valves to vent the compressed air, otherwise the compressed air returns a lot of its energy like a spring-- so there's some engine braking cam with engine braking valves on a truck's diesel. A feature that doesn't exist on a gasoline car engine.

Strike one.


But more importantly...

In my manual transmission'd car, when I'm in 5th gear and I let off the gas, the engine rpms stay in line with my vehicle speed. They are mated together with the clutch.

But in my automatic transmission'd car, let's say I need to get to 3000rpm to get to 70mph (and I've stayed in overdrive, and didn't downshift to 3rd). When I let off the gas, that rpm needle drops like a stone to 1200rpm. Try to accelerate again, and it needs to quickly rise back to 3000rpm before I start going faster.
It is NOT tied to the wheel speed.
What I've read/been told, and it make sense, is that the torque converter as designed is also a one-way fluid clutch. I've got a very basic torque converter from 1990 without locking overdrive, if that changes anything.
This is designed such that the pump must always be going faster than the turbine to drive it. The turbine cannot drive the pump: that is to say, if you let off the gas and the drive wheels are still spinning at whatever rpm is 70 miles per hour, they won't be able to drive the engine in the other direction. It's a one-way fluid coupler.

I mean, I'm pretty sure I'm seeing this every time I let off the gas on my automatic and rpms plummet right to idle, and every time I let off the gas on my manual and they don't drop any more than vehicle speed.

But that means that in my automatic, if the rear wheels cannot drive the engine in the first place (not to mention that the engine has no engine braking release valves),
I will get NO engine braking out of my automatic, no matter what gear I choose to force it into? The wheels cannot drive the engine, not in 1st, not in 2nd, not in any gear. Only the engine can drive the wheels.

I feel that this is right.
Going down pikes peak, my one and only experience with mountain driving to relate to, it seemed like engine braking wasn't doing anything and it was all due to the brakes. Let off the brakes, and I sped up as fast in forced-2nd as I did in overdrive, with nary a rise in engine rpms (seemingly untied to vehicle speed).

Maybe more modern, more sophisticated torque converters can do more. My sister feels that her 2010 nissan versa with CVT does do engine braking for her, although she may be wrong. I can't find whether it has a torque converter or not. But even if it does, maybe they lock the torque converter to use it for engine braking. Or maybe she's just imagining it. I'm curious about more modern cars/ CVT.

But it is it correct that my old 1990 ford with its 4 speed AOD and very basic non-locking torque converter, is by design incapable of any engine braking?


Apologies for the wall of text.
I wanted to lay out all of the information I believe I have, so you can follow my train of logic-- and if I can in fact engine brake, pinpoint where I've got some bad information.
I'm curious about this.

 

[SHOZ]

Air dissolved not under pressure will not be of any consequence. It is the water that will boil that produces steam that is the problem. But this is not normally seen unless you are racing or driving in the mountains.

Changing brake fluid is more about getting the corrosive mixture out of there for the most part.


As far as the engine braking and a non locking TC, shift the car into a lower gear. Does it not slow down faster? This is engine braking because the engine crankshaft is still tied to the TC and it is still pumping. In effect the force driving the pumping is the inertia of the vehicle. And you are turning the engine into an air pump as well as churning the fluid in the TC.