5 is hydrophobic. An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint. DOT 5 brake fluid is not compatible with anti-lock braking systems. DOT 5 fluid can aerate when the anti-lock brake system is activated. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air. DOT 5.1 Lack of acceptance of silicone-based fluids led to the development of DOT 5.1, a fluid giving the performance advantages of silicone, whilst retaining some familiarity and compatibility with the glycol ether fluids. DOT 5.1 is the non-silicone version of DOT 5, defined by FMVSS 116 as being less than 70% silicone. Above that threshold makes it DOT 5. Citroën hydropneumatic suspension In the 1950s, Citroën introduced a hydropneumatic suspension system, powered by an engine-driven pump and also used to operate the braking system. This used a Citroën-specific hydraulic fluid. The first fluids were of variable chemistry, and available from various suppliers. Shell Donax D, Lockheed HD19, Castrol HF were some of them. Citroën then attempted to improve and standardise the fluid in 1962 with LHS (), a vegetable/synthetic based fluid. In 1964 this was improved with the fully synthetic LHS2. In 1966 Citroën introduced LHM (), a mineral fluid. LHS was hygroscopic and gave problems with internal corrosion. Although the two fluids are incompatible, LHM has been universal since 1967, and some older cars have been converted to use it. This system was also used on Rolls-Royce and some Maserati models. Hydragas and Hydrolastic suspension Hydragas and Hydrolastic suspension were a widely used form of hydropneumatic suspension, designed by Alex Moulton, and used on British Leyland cars from the 1960s. This system was not engine-driven and did not involve the braking system. The fluid was a low viscosity fluid based on diluted alcohol. 49% alcohol 49% distilled water 1% triethanolamine phosphate (surfactant) 1% sodium mercaptobenzothiazole (stenching agent) Characteristics Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly. Viscosity For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme cold. This is especially important in systems with an anti-lock braking system (ABS), traction control, and stability control (ESP), as these systems often use micro-valves