When you say "problem" could you describe. You are talking of automotive brake fluid?
The major problems with brake fluid are a result of its hydroscopic nature- it absorbs water which lowers the boiling point and leads to system corrosion.
Lowering of the boiling point can lead to sponginess (the same as air in the system) under continuous demanding operation.
The presence of water will cause corrosion and ultimately leakage and system failure.
Contamination with mineral oil (this could be by top-up or lack of cleanliness in overhaul(even oily fingerprints can cause seal swelling) can lead to rapid failure. Mineral oil will swell and breakdown seal very rapidly.
All these problems can be avoided by 1) scrupulous cleanliness when working on brakes, and 2) regularly flushing the system (regularly means every 12-18 months).
From your description of the sample (black with sediment), it looks like your first step should be to thoroughly flush your system with brake fluid (remember, do not store brake fluid but buy as required).
I do not agree with suggestions to analyse the fluid as a first step. If you were my customer, I would recommend the above before accepting a sample for analysis because I know how difficult it is to obtain a definitive answer. It will also cost you an arm and a leg; flushing is a lot cheaper. The most critical property is the boiling point and most of the commercial labs are not setup to do this routinely. FTIR ca be used to detect certain types of contamination but it not a solve all. ICP can confirm corrosion but you already have a good indication from the appearance that this is occuring.
Hope this helps. Col
