<<when the piston is pushed against caliper the fluid is compressed.>>

Well, we are back to your understanding of physics again, are we ????
Fliuds DO NOT COMPRESS ..that is why they use them.....if they compressed , you would have ANY brakes !!!!!!!

I will only advise those here interested in changing their brake fluid of a few basics .

The main reason for fluid change is that brake fluids have a problem that inherently effects the wear and saftey to your brake system. This property
is that the fluid is 'Hygroscopic' . by definition, it absorbs MOISTURE...
..and moisture is the enemy of the internals of the brake system, along with the fact that it will boil at a lower temp than brake fluid not contaminated w/moisture.
So, manufactures recommend a simple change of fluid every couple of years
to keep the fluid safe, fresh, clean, and moisture free.
That's all, just like any fluid change..[ Some of this confusion can be attributed to US manufactures , as they never stressed this until of late, whereas the Euro guys always did ]

Now , lets understand the fluid res. tank of this post... This is a fluid tank that holds extra fluid . As the brake pads wear down , extra fluid to the brake system is needed to keep the system full . The res. fluid is then used to make up for the increased capacity of the extended caliper piston cavities. The res is not a working fluid as the fluid in the system is, it is just a res/tank of fluid to be used when the systems capacity increases due to pad wear.
So, just like any other fluid change, you want to change all the fluid in the brake system... not just the res tank...
When you change the anti-freeze coolant in the engine , you don't go change the A/F in the holding tank a few times .. you drain the block and radiator and add new coolant to the entire cooling system...... pretty close analogy.

Lastly, the brake lines go to each wheels braking unit and , because the fluid is non-compressable, the action of your foot on the pedal is transferred to a brake pad. ..and that is the end of the line..this fluid get very hot and the working of the calipers wear and add to the fluid contamination at these dead ends. That fluid never gets to make a trip back up to the res or anywhere else . It just stays there in the caliper/lines until you decide to change it by opening the bleeder and chase it out with NEW fluid that you put in the res. [ once you do one and see the crap come out at each bleeder , you will realize the importance of full fluid changes]
The turkey baster can be used to get the old fluid out of the res so that you can refill it with the new fluid you will use to push the old out of the lines with the bleeders open. but without open bleeders to vacate the old fluid, you are doing nothing but changing the fluid in the res ..the old contaminated fluid that the manufacturer wants to get rid of is still there and you have accomplished nothing more than refreshing your res tank...You have NOT CHANGED your braking systems working fluid.

I will leave it there as it is a simple concept to grasp.

brake fluid
 

my friend here at work said he bleed his brakes and destroyed his abs sensor and it cost him 300 bucks to replace it...... he says go turkey baister....

brakes
 

I have never seen anything come out the nipple at the caliper other than fluid. actually the stuff at the nipple usually looks better than the stuff in the resivor.... I open the nipple and just let it flow out in a stream... I use the old fashioned method.... (i do not advise this for you) It actually looks like a clear liquid......

Everything on Earth has movement.... Everything.....
the brake piston is in the master cylinder located on the firewall.... the caliper is at the wheel. without compression with just heating and cooling the fluid would be circulating on its on slightly. Its circulating as your car is sitting in the driveway right now... (however slight)


think about electricty.... the electron is released from the atom and it flows to the next atom causing another electron to be released and there must always be an electron to fill the gap...with brake fluid those atoms are in motions.... lots of motion.... :sun_smile

wow....
 

this is like an oil debate... I am enjoying it.

Oh , well , that put a different light on it ..

Your friend [Tech, I am sure } ruined his ABS , so he says Turkey Baster is the correct way.. and he must know brake systems , correct????... or how the hell could he scrEw uP the ABS, Right????

OK ... Turkey Baster it is ... but I know my Tech friends are going to ask
me where the hell I got that info..

I will just tell them it was "The Lone Ranger"...

its blueranger
 

its blueranger,

Look at the questions that are posted on this board just today
(which wire is positive, how to change a serpentine belt, how to open the
trunk with no battery) IF these people want to change there brake fluid then they should use the turkey baister....bottom line,


I am an old guy and I have cracked open a lot of brakes and the best way to do this is the modern way... Use a large C clamp to change your pads and use the turkey baister.....Its better if you dont crack open your system...


If you go to a professional brake place they have a hydralic C clamp they use. :D

On both my 560 and my 4.5 there is no way to use a C clamp to retract the caliper pistons. You have to use the MB tool or a big azz pair of channel locks.

Also, the fluid does look clear when it is shooting out of the bleeder, next time put a clear line on the bleeder and catch the fluid in a clear jar, it'll look like what is pictured.

Has the ABS got its own reservoir/cavity which cannot be drained by simply drawing from the bleed valves?

I also have a Volvo V70 and think have falled victim to the "do not press the brake pedal too low or you'll be wearing out the master cylinder"... I did a brake fluid change by means of opening up each bleed valve and compressing the brake pedal, down to the floor.

Now the brakes work OK, but sometimes the pedal is sost and needs a couple of pulses to get it to stiffen up.

As far as I know it, if the pedal is always soft then this means air in the system, if it is soft occasionally and after a couple of presses hard again it indicates wear in the MC. Is that right?

The hydraulic fluid is non compressible. Any dispacement in the MC is transferred to the brake calipers in a precise manner. You can lose dispacement by air in the system (gases are compressible) and balooning flexible hoses.

I do not believe there is any circulation in the brake system, there is no loop for that effect, the calipers are dead ends. Even if there were a loop, then we'd be talking about heat differentials like in gravity fed central heating systems, or we'd be looking for a circulating pump somewhere. The only way to flush the fluid from the hoses is to push it out through the calipers or suck it up to the reservoir somehow.

I have my reservation that drawing from the bleed valve renews the fluid in the caliper's piston chamber. There could be a "stream" where the new fluid travels from the brake hose to the bleed valve leaving the rest of the fluid almost intact. A better method might be to drive the piston all the way back , thus emptying the caliper of fluid, and keep it in that position untill you flush the hoses/caliper . But I think even that method leaves a lot of fluid inside the caliper unchanged.

my dear friend Arthur, I have to say that your above statement is NOT TRUE.
On the contrary, ALL FLUIDS ARE COMPRESSIBLE!!!
Since the brake medium used is fluid, it DOES COMPRESS TOO. However, the brake fluid is off-course made such that this compressibility is minimized.

This is one of the reasons for changing the fluid but not the main reason.
Afterall, the brake system is meant to me watertight and to keep moisture out.
The main reason for changing the fluid is because after lots of usage and heating up of the fluid (mostly around the caliper area), the fluid starts to loose its properties that makes it so useful such as compressibility, viscosisty etc.[/QUOTE]

Now, as far as the fluid change goes, just replacing the reservoir fluid will help in a minimal way but you are leaving the majority of the fluid in the lines and caliper area and therefore you are basically waisting your money. Flushing the fluid through the nipples is the best way.


And the mixing of the fluid in the brake system can happen through convection currents which are very minimal and do not circulate all of the brake fluid.

It is not fluid diffusion since such process requires difference in fluid concentrations which is not the case here.

just few comments
thanks.
ACM

I hate to be the engineer-geek here, but I think this is a matter of terminology. The term 'fluids' actually includes both liquids and gases. Therefore, I believe Arthur means that 'liquids do not compress.' While that is not strictly true, the compressibility of liquids is negligible compared to gases under most conditions. Liquids are commonly referred to as 'incompressible fluids' because compressibility effects can usually be ignored in analyses. In any event, I agree with the conclusion that compression of brake fluid is not a factor here.

my brake fluid is nasty.;
 

i drive a 83 300sd and the brake fluid is nasty... never been changed.

I will change it using the turkey baster and glass gar and i will catch it and you can watch it getting clearer.....

I never turn my rotors either... so I have a lip on the edge of the rotor that is probably an 1/16 inch or more.... 23 year old rotors.


I find it so hard to believe that you guys dont think brake fluid circulates....
there is a large vacant spot at the caliper that is filled by fluid when the brake is pressed and when the pedal is relaxed there is a large vacant spot at the master cylinder...the brake circulates radically until the compression is maximized.

It goes in and out but does not circulate as such. You have 4 metres worth of hoses between the two chambers, and at most you are displacing a few cm inside those hoses when the brake pads wear down to minimum. Even if you park your car uphill on a 45% slope there is nothing in the fluid to make it "rise" from the rear calipers to the master cylinder, it is all the same fluid.

I don't think I'll be buying any used cars from you. I also hope I never have to stop short when I'm in front of you.

Just how large do you think that vacant spot is? How much fluid do you think is actually moving when you push your brakes? Why don't you try having someone press the brakes, and see if you can measure how much the level in the reservoir changes, or look at the clearance between your pads and rotors and multiply by the area of the plungers. You may be surprised.

Huh? :confused:

As stated by ACM yes the brake fluid does circulate! Not very fast or much though. The reason for circulation is temperature diferences. The brakes are applied, the caliper gets hot, heat is transfered to fluid, fluid expands slightly due to higher temp. Fluid moves back through lines to resevoir somewhat. Over time yes all the fluid will be moved from the caliper back to the resevoir. But who the heck knows this time!!!! Personally the turkey baster way is the wrong way to do it. It is like draining the radiator and not the block of antifreeze and then saying you did a flush and fill! You will never quite get it all. And even over 4 weeks time you are not about to get all the brake fluid out. It just does not flow around and mix that fast. Best way is to empty the resevoir with a turkey baster, then fill with new clean fluid. Go to the farthest away caliper, normally right rear, then have wife or friend pump brakes and hold down, then open bleader until pedal hits floor, close and ket pedal go. Repeat til clear fluid comes out bleeder. Repeat on remaining three. You do not need to put a lot of preasure on the pedal since you are not trying to blast the fluid accross the room once the bleeder is open!

But I guess the pump out with baster and re-fill every week or month for a few weeks or months is better than nothing. May be half assed but it will be better than not doing anything at all to the brakes!