Article by Matt Robertson
Matt is the Managing Partner at Leland-West Insurance Brokers, Inc. He started with the firm while still a college student, way back in 1984. According to Matt his only remaining hobby is Motorsport … because its all he can afford (“will work for tires”). Reach him at email@example.com
What Do The Boiling Points Mean?
Simply put, brakes work by converting forward momentum into heat: The brake caliper – mounted on the vehicle’s suspension – contacts the rotor – mounted on the spinning wheel – and squeezes it. This heats the brakes up. A lot. Heat obviously transfers into the rotors and the vehicle’s hubs. It also transfers into the brake pads and – from there – into the brake caliper… where the brake fluid resides.
So much heat is transferred that it’s possible to boil the brake fluid in your calipers. Once brake fluid boils, it creates air bubbles and, since air compresses much better than fluid does, when you press on the brake pedal you get a mushy pedal. That pedal gets progressively worse until – if you keep boiling the fluid lap after lap – the pedal goes all the way to the floor… while you keep going in whatever direction you are pointed in. At high speed.
Typically for a passenger car you have to be doing something pretty extreme (as in “crazy”) to boil your fluid. However if you are towing a boat or trailer you can easily put yourself into a situation where your fluid is boiling and your braking is dangerously degraded. Heavy duty brake fluid is not just for weekend racers.
Further complicating the above story: Brake fluid is “hygroscopic” (commonly misspelled as “hydroscopic”, which is not a real word). That means it pulls moisture into itself from the air. No matter what you do you can’t stop this. Once you crack the seal on a bottle of brake fluid it starts absorbing moisture. Putting it into your brake system does not stop this process, and since water boils at a much lower temp than brake fluid does (a mere 212 degrees fahrenheit), the less moisture accumulated in your brake fluid system, the better as the boiling water in your brake fluid will create those aforementioned dangerous air bubbles.
Dry Boiling Point
This is the temperature that your brake fluid will boil when its straight out of the sealed bottle. When there has been effectively no time for it to absorb any moisture. If you are racing on a track, you are most concerned with this value, since the fluid in your lines is likely to be freshly added.
Wet Boiling Point
This is the temperature that your brake fluid will boil at when its been inside your braking system for a reasonably long time. How long it takes to get to this saturation point will vary by fluid type, but a good rule of thumb is the higher the dry boiling point, the faster the fluid will saturate (an informal sort of standard is – not always – used by fluid manufacturers: the wet boiling point is the reading taken after fluid has been sitting for one year in your brake lines). If you are a civilian driving on the street the wet boiling point is going to be the one you want to pay attention to since the fluid will typically stay in your brake lines for an extended period of time.
Fluid with a high dry boiling point is better suited to track use. Fluid with a low wet boiling point is less suited to long term street use. The trick is to pick the points you are comfortable with, coupled to the price you are willing to spend.
What Else Should I look For in Brake Fluid?
The information on this page is about as much as you’ll need unless you’re a professional mechanic for a racing team. The one exception to this is compressibility, and I’ll tackle that briefly in a bit. Additional variables like viscosity, rate of water absorption and recovery rate after heating are all important when you are deadly serious about racing… but thats about it. For now this page will just acknowledge those things exist – and there is very little data available to publish on the subject other than a discussion of the general principles. Brake fluid manufacturers don’t publish these specs as a rule. However, a couple of quick notes:
For the typical weekend mechanic working on a daily-driver, simply sticking to the DOT spec recommended by the manufacturer is enough in terms of safety. You won’t need to worry about esoteric things like boiling points or viscosity as the DOT standards ensure that whatever fluid you choose will meet your manufacturer’s requirements. For most people the only real question is how much money they are willing to pay for their brake fluid. You can sort the chart on price/ounce to come up with your best buy.
By and large, this isn’t something you need to worry about. However in areas subject to extreme cold, lower viscosity is better. Since viscosity is typically shown on MSDS sheets if you can find them, I may add viscosity readings to the chart above… but by and large this is a nit that doesn’t need to be picked.
While the ordinary driver doesn’t need to go here, your typical track rat knows this is a big piece of the puzzle. And if they don’t… sooner or later they’ll learn the hard way.
Fluids by their nature are incompressible. Gas by its nature is compressible (and since bubbles are a gas we are right back to not wanting bubbles in the lines). By its nature any brake fluid resists compressibility. Some fluids claim to do it better than others. The difference doesn’t have any real practical application unless you are at the extreme end of the hard-use spectrum: We’re talking laps around the race track here. With that in mind you will generally find only the top-end racing brake fluids even mention superior resistance to compressibility after boiling.
Is there a difference in the compressibility of one brand of glycol-based fluid over another? There is a relationship between the specific gravity (density) of a brake fluid and its resistance to compressibility. The more dense the fluid, the more resistance it has to compression. Fluid density is not generally reported by manufacturers except in marketing claims stating they are more dense than their competitors… without any comparative data to make the statement meaningful. Please note this article is deliberately ignoring silicone-based fluids as they are not included on the chart above at this time.
What About Mixing Fluid Types?
Your typical DOT3, DOT4 and DOT5.1 fluids are all glycol-based. You can safely mix them without damaging your braking system or compromising your existing braking performance. BUT as nearly every brake fluid manufacturer will state: doing so is not recommended. Why? Mixing good new fluid and old bad fluid means you will retain the bad fluid’s performance… its all mixed in together and the old fluid will still boil merrily away, giving you exactly what you had in the first place. You can’t fortify it with an injection of the good stuff.
So, while you won’t hurt anything by topping up your brake fluid (which you should do if it runs low for some reason), you aren’t gaining the benefits that go with a complete flush of the system.
About my Methodology:
There are a lot of brake fluid comparison charts out there on the web, on personal web pages and in innumerable forum posts (that’s how this page started out life, back in 2009). Nothing on this list got here unless I was able to confirm specs on at least the manufacturer’s site or, in the rare case that failed, multiple vendor sites. While I did find some fluids I was unaware of on other peoples’ lists, I only used them as pointers for research and did not use the specs they printed – I found a LOT of them to be inaccurate when compared to actual manufacturers’ data.
Given the above this list should be the most accurate brake fluid comparison chart on the web. I would like to keep it that way so if you have different information than I put up here please let me know what you have found.
I am not including silicone-based DOT 5 fluids as they are outside the mainstream. Think I’m wrong? Feel free to drop me a line and convince me otherwise.