DOT 5

DOT 5 is one of several designations of brake fluid denoting a particular mixture of chemicals imparting specified ranges of boiling point.

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DOT 5 is a silicone based brake fluid. It is not compatible with any polyethylene glycol based fluids. Mixing DOT 5 with other types of brake fluids will start a chemical reaction that will cause solids to precipitate out of the mixture and inhibit proper brake system function.

Unlike polyethylene glycol based fluids it is not hygroscopic, however this means that instead of being dissolved, any water in the system will eventually find its way to the lowest point in the brake line, corroding it, and potentially boiling and causing brake failure. Since it does not attract moisture there is no way this will occur unless you are careless in cleaning out of the old fluid. Its major advantage over other forms of brake fluid is silicone will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. It is widely used in the antique automobile arena as well as it will not damage paint. From a lubrication standpoint, there is no appreciable difference.

Check with the manufacturer of your vehicle to see if this voids your warranty

If you do decide to make this switch, be careful. You will have to completely flush you entire brake system of the dot 3/4 before filling with dot 5. Flushing with mineral spirits is enough to cause a complete system failure in a short time. In fact the introduction of any petroleum based products such as motor oil, power steering fluid, mineral oil(baby oil) even is small quantities will cause failure. The only way to properly flush this is with a brake cleaner that does not leave a residue. In order to flush the system properly, every component must be removed and cleaned individually, then lubricated with the new dot 5 and reassembled. You will also need to check with the manufacturer of your vehicle to see if this voids your warranty. This is the perfect time to see if any parts are worn and replace them accordingly. If done properly and no moisture is introduced into the system, you will not have to replace the internal components of your system again.

A note on Dot 3/4/5.1. The worst thing for your system is moisture, and dot 3 or 4 or 5.1 attract this. They have inhibitors which turn the fluid golden and then to a light brown before they are used up and the fluid needs to be flushed. The fluid should be checked for color and if it's darker than honey, flushed and replaced. If it's black you really need to do a complete teardown of the system and see what has corroded and replace it before continuing.