I’ve been investigating brake fluid because my Multistrada has the famous Ducati soggy rear brake. The brakes are always excellent after changing the fluid and bleeding the system, but pretty quickly, the rear brake will go on holiday, leaving a sad imitation of itself for me to work with.
Digging deep into all the possible solutions, including hanging weights from the pedal overnight to reverse bleed the system, I finally came across “Just The Way It Is” on YouTube, an experienced automotive engineer and Multistrada rider – who pointed me in the right direction.
This also explains why the brake fluid on my Tracer 9GT goes dark within 3 or 4 months, while the same brake fluid in the front reservoir remains the original colour.
But brake fluid is just brake fluid, and there isn’t much difference between them … Right?
Nope. Wrong.
Brake Fluid – The Numbers
Most brake fluids are glycol-based. These are the ones you want to use. Avoid – like the plague – anything silicon-based. DOT5 is a silicon-based brake fluid. DOT5.1 is glycol based – that is fine, just not DOT5
DOT3, DOT4, Super DOT4 and DOT5.1 brake fluids are all good but avoid mixing and matching them in the same brake system. Check the manufacturer’s recommendation for which DOT to use and go with that.
Getting Hot
Brake fluid is a liquid, and it can’t be compressed. You squeeze on the brake lever and pump a relatively small amount of fluid through the brake lines to the callipers. By pushing more brake fluid into a closed system, the pistons in the callipers, being the only part of the system designed to move, push the brake pads onto the discs.
The amount of energy that is generated as the pads rub on the disks is immense. Remember this is a 200kg motorcycle, plus 70kg or more of rider and all the gear, travelling at 60MPH plus. All the energy is focused on four brake pads with a contact patch totalling roughly the size of two credit cards.
That much energy in such a small space generates very high temperatures, and the heat transfers to the surrounding areas, including the callipers and the brake fluid they contain.
Add to this the heat generated by the exhaust and the catalytic converter, where the internal operating temperature is typically between 650°C and 870°C (1,200 – 1,600°F). At those kinds of temperatures, a lot of heat is radiated into the area around the exhaust and converter, and on most motorcycles, the rear brake reservoir is mounted not too far away.
Wet Boiling Point
With all this heat focused around the rear of the motorcycle, the brake fluid will get hot. Unlike the front brakes, where the callipers are in open cooler air, the rear brake tends to get cooked, even if the disk is smaller.
Get brake fluid hot enough, and it will “boil”. Although boiling is how it is described, simmering and producing vapour is perhaps a more accurate description. Vapour, unlike fluid, is compressible and can lead to brake fade – on the front or rear – with prolonged heavy use.
Glycol-based brake fluid is hygroscopic (absorbs moisture). Boil it, and it becomes a gas. The gas then cools, and small amounts of water are a by-product of that cycle. Do that enough times, and you end up with hydrated brake fluid (too much water) and soggy brakes, even when they are cool.
When manufacturers quote the dry boiling point for brake fluids, they are talking about the temperature at which their brake fluid, in perfect condition, will boil. 
The wet boiling point is the temperature at which the same brake fluid will boil when it contains a minimum of 3.7% water.
The bad news for us is that water boils at a lower temperature than brake fluid, which produces micro-bubbles, leading to even worse braking efficiency. All of this is why we should change our brake fluid regularly.
Although there are other chemical differences – which I don’t understand – the wet and dry boiling temperatures are the stand-out differences between the different standards for DOT brake fluids.
What is the Answer?
Not all brake fluids are created equal. The “daddy” of brake fluids is Castrol React SRF Racing Brake Fluid. Technically classed as a DOT4 Brake Fluid, its dry and wet boiling points are 325°C and 270°C, respectively, well in excess of the DOT4 standard.
Put another way, even after moisture has been absorbed, Castrol React SRF has the same boiling point as some fresh as a daisy, just out of the factory, never been heated, DOT5.1 brake fluid. It is impressive stuff.
The problem is the cost. A litre of Castrol SRF will likely cost you around £80, and a litre is the smallest size I’ve seen for sale. Compare that to standard Castrol DOT4 brake fluid, which costs about £10 a litre.
It is an expensive option unless you know a group of other bikers who want to change to Castrol React SRF and will split the cost with you. Brake fluid, once opened, does not keep. Even if you carefully retighten the lid, it will absorb moisture from the air and go “soggy”.
Motul Racing Brake Fluid
Motul has a range of brake fluids, RBF600, RBF660 and RBF700, all of which have significantly higher wet and dry boiling points than standard DOT4 brake fluid and, thankfully, at a more reasonable price tag.
As it is the wet boiling point I’m most interested in, I’ll change the rear brake fluid on the Tracer 9GT and the Ducati Multistrada 1200S for either Motul RBF600 or RBF660.
Although Motul RBF doesn’t have quite as impressive a wet boiling point as Castrol SRF, Motul brake fluid it is still far in excess of the DOT4 standard. Plus, it is significantly more cost-effective and available in smaller, 500ml bottles for £15 from SportsBikeShop. Excellent.
Clarity
I’m not a motorcycle engineer, so if you are unsure about what you are doing, please check before launching headlong into working on your brakes. The image used at the top of this article is based on a work by Philip Myrtorp and is used here under license.
Our thanks to Carl at Just The Way It Is for his assistance. You can watch his series of Multistrada videos by clicking here or click play on the video below to watch the first in the series.
