Brake fluid is a key ingredient in any cars brake system, even more so in an Aston Martin DB9 or Vantage. It’s so important that flushing the brake fluid is mandated in every annual service for the DB9 (check out the 1 year service schedule here). But which brake fluid should you buy? I set about researching this and discovered there is one disastrous wrong answer, so we need to choose wisely. Read on so you don’t make that mistake.
What is everyone worried about?
Doing a complete brake fluid flush every year seems like overkill. Brake fluid is hygroscopic, which means it will absorb water. Water from humidity in the air, or that somehow leaks past a seal, etc. Even though the brake system is a ‘closed’ system, there is still a small amount of air in the top of the brake fluid reservoir. This is a time thing, not a mileage thing. Garaging your car all year and not driving it doesn’t alleviate this necessity. It’s going to absorb just as much water sitting stored as it will in use.
Water in your brake fluid is very bad, and very dangerous. Our brakes get hot. Very hot. If you are coming down a hill and braking your brakes can reach several hundred degrees Celsius. Water boils at 100°C. See the problem? If you brake caliper reaches 150°C (which is realistic in most any driving situation) and there is any amount of liquid water in the brake calipers, the water will boil, turning into vapor, and become a bubble in the brake fluid. Vapor is compressible, liquid is not. The next time you go to stomp on the brakes all the pressure from your pedal effort will go into compressing the vapor bubbles, and not generate enough pressure to squeeze the brakes on. This boiling of the brake fluid is what we are worried about.
So changing our brake fluid with the annual service will keep the water absorption issue at bay, but can regular brake fluid itself boil if we get the brakes hot enough? All fluids have a boiling point, and if you get your brakes hot enough, even the pure brake fluid can boil. Read on….
Brake Fluid Specifications – DOT
If you’ve ever shopped for brake fluid you’ve probably seen all the bottles showing off that they are DOT 3, DOT4, DOT5 and even DOT 5.1 now. DOT stands for the US Department of Transportaion. Manufacturers usually design their cars brake system around a fluid meeting one of these standards, and then all we have to do as consumers is buy a brake fluid of at least that standard.
Generally speaking, as the DOT number increases, the performance characteristics of the brake fluid also increase (learn more about it here on Wikipedia). The performance characteristic that is changing most is the ‘Boiling Point’ of the fluid. The Higher the DOT number, the higher the boiling point. In fact, here are the DOT boiling point standards [courtesy of Wikipedia]:
| Dry boiling point | Wet boiling point[a] | Viscosity limit | Primary constituent | |
|---|---|---|---|---|
| DOT 2 | 190 °C (374 °F) | 140 °C (284 °F) | ? | castor oil/alcohol |
| DOT 3 | 205 °C (401 °F) | 140 °C (284 °F) | 1500 mm2/s | glycol ether |
| DOT 4 | 230 °C (446 °F) | 155 °C (311 °F) | 1800 mm2/s | glycol ether/borate ester |
| LHM+ | 249 °C (480 °F) | 249 °C (480 °F) | 1200 mm2/s [12] | mineral oil |
| DOT 5 | 260 °C (500 °F) | 180 °C (356 °F) | 900 mm2/s | silicone |
| DOT 5.1 | 260 °C (500 °F) | 180 °C (356 °F) | 900 mm2/s | glycol ether/borate ester |
But, to achieve these higher boiling points they are having to change the fluid types and this can lead to BIG problems. Read on.
Glycol vs. Silicone
Most brake fluids until recently were Glycol based. No need to get into what Glycol is at this point (or just check it out on Wikipedia if you want to learn more). Look at most any bottle of DOT3 or DOT4 brake fluid on the shelf in your local autoparts store and it will likely state that it’s Glycol or PolyGlycol based. But they reached the limit of raising the boiling point of Glycol and to create a brake fluid that could survive even hotter temperatures they needed to change fluid types.
There are now liquid Silicone based brake fluids on the market. These are usually sold as very high performance brake fluids, at a premium price. The chemical composition of this fluid has an even higher boiling point and can be used in even more extreme environments. DOT5 brake fluids are Silicone based. Read on to see how this can be a big problem.
[Note: The newest standard DOT 5.1 is NOT Silicone based, and the chemists have figured out how to make a Glycol based fluid with the same performance characteristics of Silicone]
Glycol and Silicone Don’t Mix
Bad things happen if you mix Glycol base brake fluid and Silicone based brake fluids.
“If you do decide to convert to silicone fluid, it should be done as part of a total brake system overhaul, with freshly rebuilt or new calipers, wheel cylinders and master cylinder. Silicone fluid should not be added to a system that contains even small amounts of glycol fluid or contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge that will not bleed out.
Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge into slugs instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems that had been operated with glycol fluid. A “new” system full of silicone fluid will require very little maintenance for years.” [Reference Link Here]
You might be thinking that what would it matter if you change over to Silicone fluid when you ‘Flush’ your system. A brake fluid flush is in now way perfect – flushing out 100% of the old fluid from all the nooks and crannies in the brake system. Some of the original fluid behind the calipers will most certainly remain. Changing from Glycol to Silicone based brake fluid would require a complete dismantling and cleanout of the brakes including the calipers, antilock brake modulator, brake lines, master cylinder, reservoir, etc. A massive undertaking. It’s not worth it.
This is what I need to warn you about. If your Aston uses Glycol based brake fluid (as it most likely does) and you go and buy a bottle asking the parts guy for “the best possible brake fluid today” they might sell you a bottle of Silicone based DOT5 fluid (cuz 5 seems better than 4). Then you top up your fluid or flush your brakes, and you’ve now mixed the two and chaos ensues (with a very expensive repair to follow).
- DOT3, 4 and 5.1 are glycol based and are compatible
- DOT5 isn’t compatible with any other type
DON’T PUT SILICONE BASED BRAKE FLUID IN YOUR DB9!!!
More about Mixing Fluids
Can we mix DOT3, DOT4 and DOT5.1? Can we mix brands? The answer is generally ‘yes’ as long as they are both Glycol based fluids.
Keep in mind that if you mix DOT3 and DOT4 fluids, you now should consider you only have the performance of the lower grade. So its fine to top up a DOT3 system with DOT4. If you top up a DOT4 system with DOT3, you’ve degraded the fluid to the performance characteristics of the DOT3 fluid, even if there is just a little DOT3 in the system.
I am a creature of habit. Once I know the right brake fluid to use, my plan is to use the same fluid from the same manufacturer if at all possible. Maximizes the chances for success.
What Brake Fluid Should We Use?
It all boils down to picking the right brake fluid for our DB9’s and Vantages [ohh, what a pun. Couldn’t resist].
The first place to start is to check what Aston Martin wants us to use, and that is documented in the Owners Manual in your Glove Box. The Fluids and Capacities section at the back of my manual says to use “Castrol Super Reponse DOT4”. You can see that page of the manual by clicking here.
I also checked the official Aston Martin Workshop Manual section 6.03 on the brake system, and it to says “Castrol Super Response DOT 4 (Not
silicon based brake fluid)”. Wow – they are telling you right there to never use Silicone based fluids (check out the reference here).
Terrific, we just need to buy Castrol Super Response DOT4 brake fluid. I promptly went online to buy myself a bottle, and was disappointed to discover that you can’t get that exact brake fluid here in the USA. Castrol sells other products, but not the exact one.
Since now I was left trying to find something ‘Equivalent or better’ than the Castrol Super Response I found the official specification for the Castrol fluid online. I wanted to find out its boiling point and other specs. A little Googling and I found it (check it out here).
You can see it is a Glycol based fluid and that it has a dry boiling point of 286°C (547° F). This is significantly higher than a generic DOT4 brake fluid, and is even higher than the latest DOT 5.1 standard.
Any fluid we buy then needs to be Glycol based and have a dry boiling point of at least 286°C.
At this point you can go out and buy any brake fluid you like that meets that requirement.
Of course I’ve already done the shopping around, and here are some options for you:
- Castrol
- NONE of their regular DOT brake fluids available in the USA that I’ve looked at meet the requirement. Their best fluid maxes out at a 264°C boiling point.
They have one specialty product Castrol React SRF Racing brake fluid
- Has a dry boiling point of 320°C which exceeds our needs of 286°C (read all the specs here)
- It’s expensive. It’s available for $74 USD per litre on Amazon.com (about triple the cost of other options below)
- This would work great, but I would recommend one of the options below since they are a much better value.
- Motul
- This is where we find a good solution. Motul RBF (racing brake fluids) have the higher performance our cars need.
- I use Motul in my DB9.
- Richard Seidlitz over at Redpants.lol is also a big fan of Motul and recommends it (check out his article here).
- Motul DOT 5.1 brake fluid doesn’t have the dry boiling point we are looking for, it maxes out at 272°C. I would not choose this option.
Motul RBF 600
- Has a dry boiling point of 594°F (312°C) which exceeds our needs of 286°C (read all the specs here)
- Is available from Amazon.com for $17 USD per 500 ml bottle.
- This is the fluid I use in my DB9
Motul RBF 660
- An even higher performance fluid (more than we need)
- Has a dry boiling point of 617°F (325°C) which exceeds our needs (read all the specs here)
- It is more expensive. Available from Amazon.com for $24 USD per 500 ml bottle
Does it Really Matter?
You might be sitting there thinking that none of this really matters for your car since you don’t track the car pushing the brakes to the limit. Can’t you just grab a cheap bottle of DOT4 off the shelf? You could, but I would argue that is a mistake. We can get our brake very hot in more ways that a track day. If you ever drive on a long downhill road (coming down a mountain, etc), you will put a massive amount of heat into your brakes controlling your speed. Our cars are heavy (nearly 4,000 pounds), and our brakes get a serious workout going down a long hill. You can easily reach very high temperatures in this situation.
My recommendation – Motul RBF 600 is the way to go. It’s not that much more expensive to use than the cheapest DOT4 generics – so use it and rest assured your fantastic cars brakes will always be there for you.
What Do You Use?
What brake fluid do you use in your Aston Martin? I’ve posted this ‘Poll’ so we can start to see what the majority of us are using. Please click on what you use, or are about to use.
Are you using something else that I didn’t discuss? Some other story to share? Please share down in the comments below so we can all learn about it.
Video
If you are interested in learning a bit more I had a discussion with Mike from Bamford Rose to get his input as well. Check out the video here.
Aston 1936 
Part of the issue with the DB9 is the location of the master cylinder in the engine bay. As it was explained to me by a tech at Aston Martin, these cars not only suffer from the heat generated by the normal operation of the braking system. But the master cylinder absorbs heat from the exhaust system which is located directly underneath it. This one two punch is why the fluid needs to be changed annually.
LikeLike
Hi Chris, thanks for sharing. Yep, the master cylinder is packed in there near the CAT and exhaust manifold. Hot to be sure.
LikeLike
Is there a way to “draw” the existing brake fluid out without a complete flush, or is this a really bad idea?
LikeLike
Hi Ken. I don’t think so. You can use a Mityvac and you could empty the master cylinder reservoir easy enough, but that’s just a small fraction of the system. To me, the only safe and correct way to do this is a proper full bleed. No shortcuts here, its your brakes. Thanks for asking though….
LikeLike