BRAKE FLUID GUIDE
Understanding Brake Fluid Ratings
DOT ratings are US Department of Transportation minimum specifications for brake fluid.
DOT 3 - now considered Obsolete
Dot 4 - Glycol-based fluid used as-standard by the vast majority of Manufacturers
Dot 5 - Silicon based fluid now rarely use and obsolete
Dot 5.1 - Glycol based fluid developed for use in high performance vehicles
Mastercylinder are nearly always marked with manufactures recommend fluid Specification
DOT 3 4 and 5.1 are compatible and can be mixed. DOT 5 was developed to withstand high temperature (260 C boiling point) and so gained a reputation as a race fluid. It is now rarely used and often confused with DOT 5.1. It was Standard fluid in 70's and 80's Harley and cannot be mixed with DOT 3 4 or 5.1. DOT 5 is prone to Aeration and neither accepts or disperses moisture making systems using it more corrosion prone, and requiring much more frequent fluid changes. DOT 5 has little lubricating ability and is unsuitable for Antilock systems with mechanical valves.
Glycol fluids are hygroscopic and will absorb moisture from the atmosphere, which does prevent undiluted water in the braking system which would cause problems boiling or over time corroding fitting. The fluid will usually darken over time as it absorbs water and 3% content will drop the boiling point of DOT 4 for example from about 230 C to 150 C and DOT 5.1 from 270 C to 190 C. DOT 4 and 5.1 contain corrosion inhibitors which cope easily with the fluids moisture content. Most manufacturers recommend the fluid is changed every year or two. DOT 4 can damage and degrade Paintwork and Plastics