Brake Fluid - Replacing

[television]

Hi Malcolm,

On my 203, both outputs from the master cylinder go to the hydraulic control unit, then to the 4 calipers. I cant see any other way the fluid goes. Can you explain further?.

Peter

Nothing to think about there Peter,,just do as wiremans excellent post

 

[vijilants]

I usually do it when I replace the pads. Its a job that costs next to nothing apart from fluid when you do it yourself and when you are doing the pads you already have the wheels off and the piston pushed back with the new pads.

 

[television]

I usually do it when I replace the pads. Its a job that costs next to nothing apart from fluid when you do it yourself and when you are doing the pads you already have the wheels off and the piston pushed back with the new pads.

Good point there, why not roll it all into one job, you hands a dirty, so the perfect time to do it

 

[turbopete]

not sure if anyone mentioned this, but the moisture in the fluid is often the cause of rusted and pitted brake pistons etc and thats when your cylinders or calipers start to leak! and it does firm the pedal up ever so slightly! id change it. in fact ive changed fluid on all the cars ive had for more than a few weeks!

 

[television]

It is said that with SBC it takes 2 people and a STAR plus 2 hours to change the fluid.

I wonder what would happen if you just undid a caliper and just let some fluid out, and then topping up againn

 

[louwdj]

Probably a bit of both marketing/fiction and fact. Does anyone have contact with any of the brake fluid manufacturers?

I was told what djb said. With hard braking the absorbed moisture turns into steam and hydraulic effect is lost. That is, no brakes. Once cooled down, brakes are normal. Difficult to prove post accident.

I have also seen cars driven with very old brake fluid, not sure if they will survive high speed, intensive braking. I have also seen slave cylinders corroded from trapped water.

I have a ten year old car, fully dealer serviced and they never changed the brake fluid for the PO and I will be checking the condition of the slave cylinders shortly.

I also believe in dismantling the slaves as the bleeder nipple normally sits high and all the crud does not always bleed off.

Although we have low humidity, I rather change regularly - around every 3 years and be safe, rather than sorry. With a pressurised bleeder its not such a chore as it used to be.

 

[wireman]

A little more info, This time about steam in over heated brakes.

There is none.

When water mixes with the fluid it lowers the temperature at which the fluid boils, the water and fluid are inseperably mixed in a eutectic bond and what you get when overheating it is a vapour of the entire fluid, It is this vapour that will cause loss of braking capability if it ever occurs (and you will have to try very hard to make happen) due to the incompressible nature of fluid being lost, as already said its not steam and does not have the same thermal characterists, what's more you can not just boil out the water - just as anti freeze mixture, brake fluid is a eutectic mixture of many components and has its own properties (dependent on the ratio of fluid and water) which differ from any of its constituent liquid's properties.

Expect boiling temperatures around 300+C for clean new fluid and lowered to 200C or so for old fluid which has been in service for many years, the fluid has become something different to what its supposed to be, it does so by nature, how much it degrades will vary over a considerable range but degrade it will and the only sensible way to deal with this is to change the fluid.

Since the heat energy dissipation capacity of any object (in this case a brake caliper) follows the square of temperature, reducing the fluids boiling point to about 0.7 (say 210C) of its original value, the heat capacity before boiling occurrs is one half or so of its intended value, this means that your brakes may only be able to do half the work that they were designed to do if the fluid is old and tired.
I believe this to be a signifcant reduction of braking system performance and a situation that is very easily avoidable, criminaly neglegent springs to mind for those who insist that it wont happen to them or does not matter.

One really good blast around the country side followed shortly by an emegency stop.....

 

[turbopete]

this might be a silly question, but whats SBC?

 

[Number_Cruncher]

>>Since the heat energy dissipation capacity of any object (in this case a brake caliper) follows the square of temperature,

That doesn't ring any bells with me - where's that dependence come from?

 

[Myros]

I was editing some photos and found this

the mess the caliper seal had made on the sl, just after it gave way. This is in the time it took me to get the camera from the house and take the photo, about 5 mins. The front section of the reservoir was already empty by then.

 

[television]

the mess the caliper seal had made on the sl, just after it gave way. This is in the time it took me to get the camera from the house and take the photo, about 5 mins. The front section of the reservoir was already empty by then.

How easy to take the brakes for granted, that picture makes you think and worth 1000 words

 

[pomm001]

How easy to take the brakes for granted, that picture makes you think and worth 1000 words

Just to add my bit
Malcolms right when he says that SBC brake flud change is about £100, i know i have one !
As far as the actual need to have it done, as the previous threads have mentioned brake fluid does absorb moisture, and as a saftey critical system it makes sense to replace the fluid at some interval. The manufactures dilema is how to determine this shedule, driver A may well live in the Alps and his surname may be shumacher so lets have a 'reasonably' short duration considering how stable new fluids are, its also a 'nice little earner' for the dealerships who's margins are constantly squeezed by the manufactures, after all who is going to arguee the toss in the dealers when its a 'saftey'issue,
and lets be fair if anyones brakes did fail and it was found to be a failure of the fluid then the driver would sue the pants of MB
Its a catch 22 situation, i think the dealer charges too much for doing it, star or no star its a reasonably quick operation, if you want to keep your MB services stamps you have no option.

 

[wireman]

Where does the square law bit come? from an answer of sorts,

If you make a body which dissipates heat 1.4 times hotter it will shift heat at 1.4 times the original flow of heat, because this heat is 1.4 times hotter and since 1.4x1.4 = 2, there will be twice as much energy being shifted. This assumes that all other variables such as cooling air flow remain constant.

Consult Wikipedia for loads more on brake fluid and some good links too.

 

[Number_Cruncher]

I can see how for conduction, there's a linear relationship;

Q=-KA dT/dx (Fourier's law of conduction)

or in a discretised form

Q= (-KA/dx)*(T2-T1)

i.e., the heat flow rate is dependent upon the temperature difference.

I can see for radiative cooling, it's proportional to the 4th power;

Q= A F e s (T1^4 - T2^4)

A - area
F - view factor
e - effective emmitance
s - Stefan Boltzmann constant

As the energy lost or gained is simply the intgral of the power (Q) with respect to time, and for a steady state, that's just Qt (where t is time, not temperature!), and so, there's no way to change these temperature dependencies.

Is convective cooling any different?

 

[television]

Is convective cooling any different?



It should be, as air is directed in a certain path with convection

 

[Weather Man]

Basic physics -- The rate of creation of heat energy and the heat flow rates are independant of the fluid, that quantity is a fixed characteristic of the system as a whole -- rotors, pads, calipers, applied force, rate of travel, hoses full of fluid. A given braking force will generate the same quantity of heat. [dQ = 1/2 mv**2 (K.E. of motion.)]

The difference must arise in the effect that heat has upon the system. If old used brake fluid evaporates more readily it will do so with a lesser quantity of heat input. A second-order temperature dependance is not obvious. Temperature change depends on heat capacity, a characteristic of the substance. dQ/dT = c

 

[turbopete]

ok, this may be a silly question, and im guessing ist on newer or high performance variants, but whats SBC mean and how do you know if a car has it?

 

[television]

Sensonic brakes were used on the 230 up to the end of 2008,
and they 211 up to around 2007.

It is a system fly by wire with a pump that keeps the pressure charged at all times, a slight touch on the pedal releases the pressure to the calipers.

No car uses it today

 

[turbopete]

obviously wasnt the best idea ever then! at least my 210 and dads 203 havent got it! i may have to be careful if i ever buy a 211 though!

 

[Blobcat]

Sensonic brakes were used on the 230 up to the end of 2008,
and they 211 up to around 2007.

It is a system fly by wire with a pump that keeps the pressure charged at all times, a slight touch on the pedal releases the pressure to the calipers.

No car uses it today

I think they stopped with the October 06 face lift on the W211. I don't have them although I do have "Brake Hold" which is a nice option and one that would be hard to not have once you're used to it.