There is a lot of misinformation about which Brake Fluid is the best to use so thought i'd post this to clear a few things up!
The trouble comes when people start quoting DOT ratings with sweeping statements like a DOT 5 or DOT 5.1 fluid is better than a DOT 4 fluid, well I'm afraid you can't make statements like that because in most cases it isn't true, you get lots of opinions about whats the best brand so here is my opinion based on some facts!
Firstly to set a few things straight.
Castrol SRF is DOT 4 rated and is definitely not DOT 5.
DOT 5 rating is reserved SOLELY for Silicone based brake fluids and these should not be used in performance applications. Basically any moisture entering the system will stay in small bubbles or pockets as water cannot be absorbed into silicone fluids. As soon as the fluid reaches around 100C (pressure dependent) the trapped water will boil. As gases can be compressed and fluids can't your brake pedal will become softer. Separated water in the system can also lead to corrosion on metal components (pistons etc.). Silicone fluids should not be mixed with other DOT rated fluids as they are completely different chemically.
DOT 3, DOT 4 and DOT 5.1 brake fluids are Glycol-ether based fluids. Glycol-ether is hygroscopic which means it absorbs water into the fluid (water combines with the fluid, water doesn't sit separately as with Silicone fluids). As water is absorbed into the fluid it will gradually reduce it's overall boiling point. For this reason fluids are rated with with two boiling points, one with no water absorbed into the fluid (Dry Boiling Point) and one with 3.5% water absorbed into the fluid (Wet Boiling Point).
The Wet and Dry boiling points are the MOST important factor when deciding what brake fluid to use, DOT ratings set the minimum requirements for these two boiling points. The higher the DOT rating, the higher the minimum boiling point requirement.
HOWEVER that is not the whole story as the DOT requirements also stipulate other criteria that the fluids must pass including acidity, corrosion protection etc. but the most important one for us after boiling points is viscosity.
With the introduction of high tech ABS/TCS/ESP systems there was a need for thinner brake fluids so they could pass easily through the micro valves used in these systems. The main reason why the DOT 5.1 rating was introduced was to set a standard for these thinner fluids.
DOT 5.1 criteria dictates that the fluid will be thinner in viscosity than a DOT 4 fluid.
If your manufacturer says you only need a DOT 4 rated brake fluid (as mentioned above, all BMW's only need DOT 4) then you don't need a DOT 5.1 fluid. In fact there aren't many cars that specifically require a DOT 5.1 fluid.
If you check the Wet and Dry boiling points of the most popular DOT 4 and 5.1 fluids you will find the best performing fluids (like Castrol SRF) are in fact DOT 4 rated. The reason they are DOT 4 rated is they are too thick to pass the DOT 5.1 viscosity criteria even though they surpass the DOT 5.1 wet and dry boiling point criteria by miles!
The really important numbers (to me anyway) are the Wet boiling points, SRF wins hands down with this and you can see why it is so widely regraded as the ultimate brake fluid. What the wet boiling points don't tell you however is how quickly the brake fluids can absorb moisture, some fluids are more hygroscopic than others and will get to the wet boiling point quicker than others. Manufacturers can get around this by treating the fluid with Boric acid to help neutralize any water entering the fluid and makes the fluid slower to drop to the wet boiling point. Castrol use this method with their 'Super Dot 4' giving it a longer life. ATE 'Super Blue' is also a very good performer over a long period and I believe was the OEM fluid for BMW's for a while (I have used it in the past and it's been excellent).
Whatever fluid you chose change it AT LEAST every two years and preferably annually.
SRF is expensive and the next alternative is Motul RBF 600 at half the price its worth considering when SRF is approx. £40/Ltr. especially if you do change every year its add's to the already high running costs we all experience...!
A few general tips about brake fluid...
Only buy enough fluid for what you need at the time. Fluid sitting on shelves just absorbs moisture from the atmosphere so when you come to use it, it maybe no good. Incidentally that is why decent fluids come in metal cans (ATE for example and SRF used too), moisture cannot pass through metal where as it can permeate through plastic.
Speaking of moisture permeating through plastic, tests have proven that the most moisture entering a brake system comes from...moisture permeating directly through the rubber of the brake hoses! Remember that the next time you drive through a puddle
The Importance of Braided Brake Lines
Installing stainless steel braided brake lines should be one of the first purchases for ANY sportscar. Replacing the crappy OEM rubber brake lines has two major benefits: eliminating brake fade and reducing your maintenance. This will help to reduce water permeating through the rubber only hoses, braided are still rubber in the centre so there is still the possibility water getting in but its deffinitely reduced with braided lines but not eliminated.. there are plenty of cheap Halfords sheit out there so make sure you spend a few bux and get DOT approved ones!
Safety first: Eliminating dangerous brake fade
Brake fade is the loss of braking power when your brake system overheats. When you apply your brakes, brake fluid is forced through the rubber brake hoses, squeezing your brake calipers and causing you to decelerate. A by-product of the braking action is HEAT which is bad for your brake lines. As the brake fluids heats up, so do your OEM rubber lines. Hot rubber lines will expand when the brakes are applied - instead of channeling the fluid towards your calipers, they expand outwards, robbing you of valuable braking power.
Stainless steel braided brake lines don't expand when heated by your brake fluid. You will get maximum braking power regardless of the time spent riding. These brake lines may save your life one day.
No Maintainence & Peace of Mind
Some manufacturers recommend their OEM rubber brake lines to be changed every couple years. Most stainless steel brake lines never need to be replaced - Most cases one purchase and guaranteed for life!
Thanx Mroad For Info!
