Let’s be clear up front – if your brake fluid has not been changed in over a year, you are past-due to replace it. Brake fluid is hygroscopic- it naturally absorbs water from humidity present in the air. Therefore, it requires regular maintenance.
This article is meant to help you make an informed choice when you hit the parts store.
A lot of technical innovations in automotive chemicals have been linked to better performance, increased engine or component life or a less toxic, more earth-friendly disposal for spent fluids. Brake fluid – the hydraulic liquid that is used to “push” your brakes when pressure is applied – has similarly gone through evolutionary development.
Very much like engine oil or power steering fluid, there are dozens of brake fluid choices at your local parts store. They carry everything from the store brand DOT 3 fluid costing a buck, to very high end synthetic fluids that may cost twenty dollars per can. Keep in mind, the same rules that apply to selecting engine oil apply to brake fluid: the more expensive option is not always the best option.
Some of the brake fluids available for racing applications might perform well when used in those conditions but might not be right for the street.
So, what is the difference between DOT 3, DOT4, DOT5 and most recently DOT5.1?
The US Department of Transportation (that's what the DOT stands for) established specifications defining a number of PROPERTIES to which brake fluid must adhere without defining chemical composition. Those specifications relate to boiling point of the fluid, (both dry and wet) how viscous (flow able) the fluid is, and stability of the fluid at high temperatures among other properties. Succeeding generations of DOT fluid standards have raised the minimum boiling point. By the way, "DRY" designates new, unused brake fluid with 0% water. "WET" fluid as measured for the boiling point standard has absorbed up to 3.7% water.
While we make efforts to keep brake systems impervious and “dry” over time, even a buttoned up brake system with tight seals and new lines absorbs some moisture. The key here is what happens to that moisture after it enters the system.
Absorption of water from humidity over time lowers the boiling point, making it more likely the fluid will boil. Picture summer driving in the city. The constant stop/start in traffic gets your calipers extra hot. This allows the fluid to boil as it reaches hot calipers. Gas or vapor formed when liquid reaches its boiling point allows the fluid to "compress" making for longer travel when you apply the brakes. Typically, people describe this as having a “soft” brake pedal. In the worst of these situations you may need to "pump" the brakes to have them take action.
The most common brake fluids - DOT 3 fluids are primarily glycol ether; DOT 4 fluids are also glycol ether based, but have borate esters added to increase the boiling points. DOT 5 fluid was manufactured using silicone which does not absorb water.
The point behind creating a silicone based fluid was to avoid water absorption. Unfortunately, as we described in this article cautioning against using DOT 5 Brake Fluid in a new Master Power Brakes' disc brake kit, water still gets into the brake system, pooling or puddling rather than being absorbed into the fluid. That leads to corrosion in the system.
Most folks know they aren’t supposed to top off DOT 3 or 4 brake fluids with DOT 5, but don’t know why. The answer goes back to the chemistry. Combining even trace amounts of a glycol-based brake fluid with DOT 5 can cause the two incompatible fluids to gel, resulting in poor braking. Converting to DOT 5 also requires thorough flushing and removing ALL traces of the old fluid to avoid seal damage. For all of these reasons, we do not recommend using DOT 5 brake fluid with our brake kits.
Let’s review: Brakes get wicked hot especially under extreme conditions. When the calipers (and the fluid reaching them) get hot that fluid can – and will – boil. Boiling produces gas which is more compressible than the fluid leading to soft, spongy brake pedal feel and a longer travel time when applying the brakes. As water enters the over time, hygroscopic brake fluid begins absorbing water from the atmosphere. Brake fluid containing water boils with less heat.
Our recommendation to customers buying our drum to disc brake conversion kit is to go with a high quality DOT 4 fluid such as the Pentosin Super DOT 4 Synthetic fluid. The higher quality fluids offer a chemical makeup that is more resistant to moisture and contain the proper rust inhibitors we need for our classic cars.
Now, to wrap up, let’s talk frequency of changing out your old brake fluid. Put it on your once yearly maintenance list and you are likely pretty well covered. A cautionary note here- you should change DOT 4 fluid more frequently than a DOT 3 fluid, because water will be absorbed more quickly in the DOT fluid.
As always, if you have questions about any aspect of your brake system or if you need ideas on how to improve the brakes on your classic car, give us a call or drop us a line. Our entire team is crazy about old cars and we’ll gladly lend a hand!