All,

Finally had chance to upgrade to Castrol SRF today. To my horror, the shop tech flushed out the stock brake fluid and you wouldn't believe it unless you saw it for yourself (picture attached below).

Brake fluid is supposed to be a golden amber like beer, what came out look like black sludge from the tarsands of Canada lol. As I has suspected, the fluid had not been changed in years, and the braking performance showed it. Any car that has not changed its brake fluid in the last 2 years will have similar looking brake fluid, and it will compromise braking performance more so than most people realize. It is by far the #1 most overlooked maintenance service of all.

I flushed the brakes and the clutch line and the effects are definitely noticeable as far as pedal feedback in both the brake pedal and the clutch pedal. Both the brake pedal & clutch pedal are about 10% firmer and the brake feedback is more linear and crisp than on the stock old fluid. Did a couple of very hard brakes in a row and there was zero brake fade. The clutch pedal feels much more like a conventional manual and definitely feels more like an aftermarket sporty clutch than stock (in a good way). Always felt the stock pedal was just a little too soft.

I was hoping it would alleviate the clutch pedal situation after a long hard high speed brake, and it did help a bit, but it still did not cure the problem completely. I know all the vantage brake lines are stainless steel, however I suspect the clutch line may be made of conventional rubber. The brakes never fade no matter how many stops I do, but the clutch pedal goes soft. So although they share the same reservoir something else is going on.

On the Ford Mustangs (which is the other car that most commonly exhibits this exact symptom as well), the forum guys claim its because the slave cylinder is so close to the headers that it gets super hot after a really long hard pull (2nd gear - 5th gear) thereby causing the fluid/hose to get too hot & soft. Usually after 30 seconds the everything returns back to normal every time leading me to believe the heat issue theory may have some legitimacy to it.

Not sure if the cure for this is a stainless steel hose or something else entirely, will need to investigate further...

Overall I am very happy with the change, even though it did not solve the high speed clutch pedal softness completely, it did help it and also significantly improved clutch pedal firmness and brake pedal feedback under normal driving. In addition, I performed some much needed maintenance with the peace of mind knowing I put liquid gold in the place of the stock black sludge.

Hope that helps,
007

 

Great post. I wonder if the discoloration is caused by the degradation of inside of the brake/clutch hoses.

 

Hey Aerome,

The discoloration is actually moisture (water) getting into the lines and breaking down the fluid. Has nothing to do with rubber. The stock brake lines are not rubber anyways, they are stainless steel mesh lines so it's not that. Brake fluid is hydroscopic (seeks water) so anyway it can get moisture it will. After roughly 1-2 years all average brake fluid looks like this, it has to constantly be replaced (which nobody ever does). Castrol SRF repels water the most out of all other brake fluids which is why it has by far the highest wet boiling point. Under normal city driving you could go 2-3 years probably without any moisture in the lines, under racing conditions at once a year is necessary. SRF is also 10% less compressible hence why it provides much more direct pedal feedback (less mushy pedal feel)

 

Why do you say its less compressible? unless it has suspended air in it, its a fluid and follow fluid principles of non-compressibility .
as far as the brake fade goes. the fluid doesn't partake in this either, unless you have so much air in the line that you lose your pedal completely. fade is really only a factor of the pads losing their grip on the rotors.
Now, stainless steel lines, still use rubber cores, very similar to the stock lines. the difference is that they are wrapped in stainless steel sheath.
the discoloration is oxidization and breakdown of the fluid and break lines.

 

Certain brake fluids are less compressible than others due to their properties. Not all hydraulic fluids are exactly the same, just like not all motor oils are exactly the same. Some are just formulated that way. Even when cold, they are less compressible thereby giving improved pedal feedback (this is common knowledge when it comes to racing brake fluids).

Fade is a totally different topic, but yes when hot air bubbles form pedal does get soft. However, this has nothing to do with the compressibility of the fluid, it's the fluid actually boiling into a gas. With that said, SRF has the least amount of fade of any brake fluid on the market and it can bounce back after extreme heat situations (which is why Formula 1 ran it for so many years). Any other brake fluid (include Motul or super blue)... Once the fluid gets hot and boils over, it's completely done and must be flushed. For that reason you can track in SRF all year long and just flush it once a year, no other brake fluid can do that. It's also why it's so much more expensive than any other brake fluid.

Also, lots of stainless steel lines don't use rubber cores, many can use either silicone or Teflon or some other higher temp synthetic material depending on different applications.

 

Yes, fluids are 'incompressible', but the degree to which they are incompressible is not 100%. Some fluids are better than others in this regard. DOT 5 for example, is more compressible than DOT 4, so it gives a slightly spongy feel in comparison, but it doesn't attack paint, so it is often used for collectible vehicles where absolute brake performance is secondary to protecting the car's (or bike's) finish from accidental damage.

And SS lines typically use a teflon tubing which resists expansion upon application of internal pressure. None use rubber lines; that would be pointless.

 

Is all this even worth it?? The OEM fluid is Castrol React Performance DOT 4, which meets DOT 5.1 boiling points..(hope you landed a good price per L)..Because the cost vs efficiency doesn't really play out when I look at the specs..I mean even the GT4 series use React Performance and I'm sure there systems will see a lot more heat than road/weekend track vehicles....just saying

btw: brake fluid color is not amber beer looking like after 2 years of regular driving...only beer it's going to look like is Guiness(which is awesome btw)..

 

Now hold on a minute. your bringing up several things here, and not all are going to determine pedal "feel".
Lets get the terms straight:
wet boiling point and dry boiling point.
almost all good racing fluids have great dry boiling points. (right out of the bottle.) so, YOU WILL NOT FEEL ANY DIFFERENCE after you bleed the system and go drive. after a year or so , or any racing, you might. this is the "wet boiling point" effect. this is how much water the fluid will absorb.
fluids are INCOMPRESSIBLE. now, if its not a pure fluid, then there is a factor that is compressible, but generally, its very very low. way too low to feel. so low, that the manufacturers talk about it, with no facts to prove it. (they are talking about dissolved air in the solution, which is a very small amount). get a density figure for the fluid and that will rank the fluids for their compressibility. (specific gravity number)
Now, I do race and have for over 17 years, so I hear all the padock talk and have used it all and raced cars that are powerfull and hard on brakes at some of the toughest tracks in the world.
if you change your brake fluid and your pedal feels better, you probably just got the air out. if you do the same with brake lines (stainless vs rubber), same thing! brake lines don't bulge like folks think. most are extremely rigid when in good shape. the stainless protects the lines. that's the main function. Teflon and other materials can be used with braided stainless as inherently, are not as maneuverable as rubber and needs a special compound internal, with an external casing vs pure rubber lines.


Now fade. air in the lines or bad fluid , doesn't give fade. it only makes the pedal go to the floor or make you require more force to get the same braking pressure on the rotors to stop/slow your car . fade is a condition of brake pad heat or lack of grip. this is dependant on the materials and temps of the rotors and pads. this is caused by boiling the fluid, where air bubbles are created. all fluids will boil with the associated temp applied to the fluid. ALL fluids. rotors get near 800 to 1000 degrees, calipers can get over 500 degrees, and if the boiling point is under that temp of the caliper, specifically the rotors touching the pads, which touch the pistons, which are in contact with the fluid, you can get boiling. the trick of racing brakes and the drivers is to create that heat for such a short time, that it doesn't get transferred to the calipers and then to the fluid. I can boil any brake system, but I can also drive at the limit and not boil it either..... that's a racing skill set.


you can boil racing fluid, you will get a soft pedal and then bleed it and it will be like new again. letting it sit until cool can sometimes have the same effect, but generally, its better to bleed the lines to get that air out.


the advantage of SRF is like you said, it has less propensitiy to absorb water from the air, and has the highest wet boiling points. (means you can leave it in longer) however, once you boil it., you have to bleed it to get the air out.


by the way, in Europe, most of the race teams are using Endless now, which is not much better wet vs dry than superblue ATE . 400/500 vs SRF 500/550, and ATE 360/500. so, you are right, SRF is certainly the best of the group, but also is 4x more expensive than the "standard" racing fluids. I just ordered a bottle, so im on board, but might have to order another for my race car!

 

Fluids have different compressibility; they are NOT 100% incompressible as you seem to think. I guarantee that you can feel the difference between DOT 5 and DOT 4 right out of the bottle, especially on motorcycle front brakes because of your very sensitive fingers.



Compressibility of Liquids

Liquid
Compressibility: kPa-1 x 10-11

Carbon disulfide 93

Ethyl alcohol 110

Glycerine 21

Mercury 3.7

Water 45.8

Compressibility is the fractional change in volume per unit increase in pressure. For each atmosphere increase in pressure, the volume of water would decrease 46.4 parts per million. The compressibility k is the reciprocal of the Bulk modulus, B.

 

are you reading what you wrote above and really understanding how it effects the braking system? 45 parts per million for each 15psi change? (and that the TOTAL for the type of fluid, not comparitive) So, and by the way, we are talking about DOT 4 to DOT 4 Glycol based fluids. Yes, there are specific gravity numbers (that speak to relative densities) and that's what you need to look at. look at the percentages and let me know how you think you can feel that in your hand or foot. You cant. anyone that has been racing for a while, knows this. the street guys say things like this. their wing give them more down force in the corners, the air filter gives them better gas mileage, etc etc.
Its not that hard to figure out. if that pedal can be pressed a 1mm (and this is being VERY generous) further to the floor with a different fluid, will you be able to feel that? because that's the reality of the most extreme density differences in fluid, based on the volume and pressures used in the system.

the real issue here is in a racing environment,when you heat the fluid very hot. the water in the systems, turns to a gas, and releases that gas into the fluid. now, it becomes much more volumatic. HUGE difference. thats when you feel a soft pedal or it goes to the floor. we are talking 5% or 10% increased volume in the system, with a compressible fluid.
Not, .001% due to the density differences.

 

All I'm saying is that fluids are not incompressible, which you've stated to be true several times. How much you can 'feel' depends on the particulars of brake design. You can easily feel the difference between 5 and 4 on MC front brakes. For cars, not so much.

 

For our discussions and the reality of the situation, the fluid is incompressible. However, I its possible that the silicon fluid might be more notable vs DOT 3. I don't know. if you can track down the science of the fluid composition differences, you can then figure out how much the difference might be as far as feel. but if the difference between water being compressed at 45 parts/million and something like alcohol being double that at 15psi....... even at 1500psi, (extreme braking pressure) the volume difference would be 100x or 45parts /10,000....... still less than .5%....... and this is with two entirely different fluid families.


So, the point is, you are not going to be able to put two types of fluid in the brake system detect its difference in compressibility. Its just not possible. Any differences would be purely imaginary, or possibly due to an uncontrolled test where one of the fluid changes introduced less air into the system. The reason that there is a fluid discussion here, is we want to know what the change interval is, and for racing fluid, as 007 says, the SRF is better due to its different Hygroscopic qualities. less water absorption, means less corrosion over time to the seals and moving parts in the system. Also, means less gas introduced during racing conditions due to the higher boiling point. Pedal feel is not something that this kind of fluid change would effect in the slightest. a little air in the system, creates a soft pedal. A little air in the system is several orders of magnitudes greater than any difference in what two different fluid would exhibit.


scientifically , it can be proved and estimated . no one here has posted the specific gravity of any of these fluids. that would be the indicator. Then, If there was a compressibility factor, and a volume estimation of the master cylinder, (as well as master cylinder max pressures), you could estimate how much pedal movement would be related to it. Without this information, its all anecdotal evidence and guess work.

 

One of the biggest problems I see here, is not the semantic debate of the fluids in street operation, but why the falling pedal on the clutch with High RPM. Is there any vibration at high rpm that might put forces on the hydraulic TB and push fluid out into the master cylinder? with the older style, on other manuf. cars, the TB slave cylinders can be pressed on by a vibrating drive shaft which pushes the slave shaft back into the slave cylinder slightly, and then the pedal goes toward the floor. I don't know if our system can do this.
I don't think its temperature, because its only happening at high rpm as I understand the description of the problem.

 

Sounds like the clutch wasn't bleed at the time of the brakes..both share the same reservoir..it's the same principle of heat with brakes in relation to fading pedal.. There is a lot of heat generated in the slave and 9.99999/10 fluid is never flushed out and replaced for the life of the car until the clutch is replaced..when that water turns to vapour I can see this simulating a falling clutch pedal, don't forget high rpm is creating huge amounts of heat from friction torque..I can't give a detailed explanation like u guys are battling out but I can state this on my experience working on them for a living and not a side hobby! ;-) BOOYAH!!!

 

its possible, but not likely. the clutch TOB is not subjected to heat that could even come close to boiling even the worst of fluids. and certainly, the heat wouldn't be transferred at higher RPM at any faster rate to cause this water In the lines to boil and create a falling pedal that comes back at the lower rpm.
How does the high rpm crate heat from what " friction torque"??? what do you mean by this?


what is causing the soft pedal is high rpm, and it has to be related to forces due to the high rpm... its not a heat caused effect.
If you are working on "them" , I suppose you are talking about AMV8s, right? if not, then you might not know what kind of TOB and actuation mechanism we are speaking about. im thinking its due to some issue in the hydraulics of the TOB itself.