Short answer
DOT 5 is the silicone-based brake fluid grade in FMVSS 116, defined as at least 70 percent diorgano polysiloxane by weight. It must be dyed purple, meet the highest boiling point minimums in the standard (260 °C dry, 180 °C wet), and it does not mix with the glycol-based DOT 3, DOT 4 or DOT 5.1 fluids. It is used mainly in systems designed for it, not as an upgrade for ordinary cars.
Silicone versus glycol
Glycol fluids absorb water and disperse it. Silicone DOT 5 is not hygroscopic: it does not absorb water, so any moisture that gets in stays as separate droplets that can settle in low points of the system. That is one reason it suits vehicles that sit for long periods with an otherwise sealed system, and why it needs a clean, dry system to start with.
Silicone fluid also does not attack paint the way glycol fluid can, which is part of its appeal for show and collector vehicles. FMVSS 116 even exempts silicone fluid from one low-temperature stratification check that glycol grades must pass, reflecting the different chemistry.
Why it is purple
FMVSS 116 requires DOT 5 silicone base brake fluid to be purple, while DOT 3, DOT 4 and DOT 5.1 must be colorless to amber. The color rule exists so a technician can identify silicone fluid at a glance and avoid mixing it with glycol fluid.
Where DOT 5 is specified
DOT 5 has been used in some military vehicles, in some older Harley-Davidson motorcycles, and by restorers of classic and show cars. Most modern vehicles with ABS or stability control specify a glycol fluid instead; silicone’s different compressibility and tendency to hold tiny air bubbles are reasons commonly given by system makers.
If a vehicle’s master cylinder cap or manual says DOT 5, it was designed around silicone fluid. Putting glycol fluid in that system, or silicone into a glycol system, should only be done with a complete flush and the manufacturer’s guidance, and some builders replace seals when converting.
Do not confuse DOT 5 with DOT 5.1
The names are close, but DOT 5.1 is a glycol-based fluid that meets the DOT 5 boiling point limits. FMVSS 116 requires DOT 5.1 containers to state “non-silicone base” for exactly this reason. Read the full grade on the bottle before adding anything.
Frequently asked questions
Can I top off DOT 5 with DOT 4?
No. Silicone and glycol fluids do not blend. Mixing them can lead to separation and seal problems; drain and flush instead, following the vehicle maker’s procedure.
Does DOT 5 ever need changing?
Even though it does not absorb water, it still collects contamination over time. Follow the interval in the vehicle’s service manual.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].