Brake fluid will absorb moisture out of the air. It is the moisture that rusts the metal brake lines from the inside. The fluid will turn dark. Plus if a vehicle is left to set for a very long time, the fluid can get algae in it and give it a greenish tint. Found that out after bringing a truck back to life. The cleaner the fluid, the better.
Originally Posted by TailHooked
Tailhooked is correct about some forms of algae being green; due to the presence of chlorophyll - that needs sunlight. However, there are other bacteria that will grow, such as the slime molds (myxomycete). The green that is seen is very likely from the corrosion of the copper and brass in the brake system, showing up in the cupric form.
The presence of the green is an absolute tip off the the fluid needs to be change because it would indicate the presence of copper corrosion due to a pH change in the break fluid due to break down of the fluid. A flush out of the system would help to get rid of the slime and biofilms that can cause several types of problems.
Diethylene glycol is a better brake fluid than castor oil used a long time ago, but it can still break down, and like all of the glycols can be digested by some of the bacteria, With antifreeze solutions of ethylene glycol or propylene glycol a drop to 20% or below, bacteria will immediately turn them into food. That can start much above 20% with a pH change. The term "permanent antifreeze" by
legal definition is up to 3 years, after that is not considered to be good anymore. Some large systems are run for decades by using pH adjustments, fluid preservatives and maintaining corrosion and biocide inhibitors; those systems do not contain aluminum which will corrode at the higher pH (above 7 is alkaline) required for stability on steel. With the advent of the aluminum block engines on cars in the 1950s a lot of things changed, including the lower pH requirements (below 7 is acid).
To make this short, it is a good idea to just change the fluids, on the tiny automobile and truck engines with less than three foot in diameter pistons and 20 plus pistons in an engine. It is a Little bit expensive to fly someone in to look at your pistons, and test any deposits, during an engine rebuilt after continually running for two or three years.