As you know, the brake fluid reservoir in a King Air is located in the upper left-hand nose avionics bay. Not filling the reservoir properly can spell trouble for some of your avionic units.
When you remove the avionics bay door, you can tell where it is because of the red stain on the dust cover. This hydraulic fluid stain is not a problem. The true problem usually lies inside the avionics equipment mounted just forward of the reservoir. In some King Airs, the Flight Director Computer is mounted here; in others, the Autopilot Computer and Air Data Computer are there; in still others, other avionics units are located there.
When the brake fluid reservoir is over-serviced and the seal on the screw-top lid is worn, that can spell a sticky and expensive situation for the sensitive electronic equipment mounted nearby, as the leaky fluid can affect these units. It is a very common issue and very easy to do. Chapter 12 says to "fill to within one inch of the top." Be sure not to overfill!
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