The first hyraulic systems - automotive brake systems - were produced about 1920. At the time there were no synthetic rubber compounds that were immune to attack from petroleum-based products, so a non-petroleum fluid had to be used. The choice was glycol.

Synthetic rubbers were developed during the WW II era that could resist petroleum-based products. They were first used on hydraulic control systems for aircraft, and hydraulic systems using petroleum-based fluids soon replaced the cables and pulleys on construction and other heavy equipment.

Because automotive brake systems originally used glycol-based fluids, the various authorities ultimately including the US government decreed that automotive brake systems continue with glycol-based fluids because of the logistics issues of having to maintain two different and incompatible fluids in the field and the potential safety issue of mixing petroleum based fluids in older systems that could cause brake failure.

Thus we are stuck with the legacy of glycol-based brake fluid and its moisture absorbtion, which will corrode the internals of the brake system if it is not periodically flushed to purge absorbed moisture. The only other fluid that is acceptable for brake systems is silicone-based, but since it has certain incompatibilites with glycol-based fluids such as miscibility, it is not recommended that the two fluids be mixed, so silicone fluid should only be used in a brake system if the system is completely disassembled and cleaned and then everthing put together with silicone fluid.

Duke