Brake Fluids

There are several types of brake fluid out there, including DOT 3, 4, 3 & 4, 5 and 5.1. The most used are DOT 3, 3 & 4 and 5. DOT stands for Department Of Transportation. If it is critical to the operation of your ride and it doesn’t say “DOT approved”, don’t use it.

The numbering system for brake fluid, 3, 4, 3 & 4, 5 and 5.1 comes from a rating given by the Department Of Transportation when they determine a brake fluid’s compressibility and its ability to resist boiling under extreme braking conditions. The higher the DOT number, the better the fluid.

To understand these numbers, you have to consider how far we’ve come with brake technology.

In the days after mechanical brakes, DOT 3 was the brake fluid of choice. It did everything a brake fluid was supposed to do in a drum brake world. It didn’t compress much, resisted higher braking temperatures, and because it is a glycol-based fluid, it also absorbed moisture to help prevent rust from forming inside the brake system.

During the 1970s, disc brakes came into vogue along with the need for a brake fluid that could withstand higher brake temperatures. To accomplish that, DOT 3 was upgraded to what we call DOT 4. DOT 4 is a higher-temperature rated version of DOT 3.

To remove the confusion as to which fluid should be used in which vehicle, there are a few things you should know. that by combining DOT 3 and DOT 4 and calling the result DOT 3&4. This brake fluid is really DOT 4, but the combination numbering seemed to remove the confusion. DOT 3 is slowly being phased out and may not be available in your area.

DOT 5 is the one brake fluid that is completely different from the others in that it is silicone-based. This fluid was developed specifically to satisfy the extreme heat environment requirements of the racing industry, which is 500°F plus. This brake fluid moved its way into the aftermarket due to its high heat tolerance and it’s being used in off-road vehicles, drag cars and other types of extreme vehicles.

DOT 5.1, was developed as a counter fluid to DOT 5. This is a glycol-based brake fluid that has the high tolerance to extreme temperatures that silicone fluids have, but unlike silicone fluids DOT 5.1 has a natural lubricant that makes it the ideal brake fluid out there for anti-lock braking system setups that use mechanically cycling proportioning valves.

Which type of fluid is best for use in vintage rides, glycol or silicone?

In the past, manufacturers have specified the use of glycol-based brake fluid in their vehicles. The choice is easy, glycol-based brake fluid absorbs moisture in the braking system, preventing rust from forming in the lines and master cylinder. This fluid is extremely forgiving. When was the last time you changed the brake fluid in your vehicle? Was it never? I don’t remember ever changing the fluid in any brake system for the sake of changing the fluid. I’ve replaced the fluid when I restored the entire system, or replaced a line or brake cylinder, but not because the maintenance log said I should. That’s the reason manufacturers use glycol-based brake fluid.

You should check to see if your brake fluid needs to be changed. Observe the plastic fluid master cylinder reservoir and determine the fluid color. Good fluid will be crystal clear. If you can’t determine the color, remove the master cylinder cover to observe the color of the fluid. This brake fluid absorbes moisture from the air, so limit the time the master cylinder is uncovered.

Brake fluid with moisture in it will have a brown tint to it. Flush the system if the fluid is tinted brown; keep on driving if the fluid is clear.

Harley Davidson is the only manufacturer I know using DOT 5 brake fluid.

Glycol-based brake fluid big disadvantage would include its ability to remove paint and corrode electrical components. It can wreak havoc on your late model Ford truck should a leak develop in the plastic reservoir on the master cylinder. Fluid will drip from the reservoir down onto the cruise control module, the electrical terminals inside the module corrode and overheat and your truck burns.

In the silicone world you have a fluid designed specifically for high heat conditions and it won’t harm your paint or hard plastics.

The disadvantage of silicone is its inability to absorb moisture. It is also more compressible than glycol-based fluids, which can equal softer brakes. It provides no lubrication to the system, is incompatible with the mechanical valving in some anti-lock braking systems, and it should be changed yearly because of its tendency to aerate.

If you shake up a bottle, it will turn milky due to its tendency to absorb air. To get rid of the air, let the bottle sit undisturbed for a while.

Whichever route you choose, be aware that glycol and silicone brake fluids are completely incompatible with each other. When changing from one to the other, flush all of the old fluid out first.

This article is the intellectual property of Simmons BOSS CREATIONS. Any reuse of the contents must include the following attribution:

Marcus Simmons, ASE Certified
Phone: (248) 461-6977