Contaminants such as rust in the brake lines and caliper/drum pistons, particles from brake masterpump will darken the color over time. However, the most damaging contaminant is moisture from the environment. Brake fluid is hygroscopic, which means it will absorb moisture. It is designed that way to prevent corrosion in the brake lines, master brake pump, slave cylinders, disc caliper cylinders.
Moisture in the brake lines will cause the brake fluid to boil near the normal boiling temperature of water (100 deg. C), versus the specification for DOT 4 (about 300 deg C). Imagine going downhill from Genting Highlands, the brakes heated up the brake fluid, and the moisture in the brake fluid starts to boil. It will greatly reduce the braking performance. For this reason, heavy vehicles, such as trailers, uses "Air Brakes", which is able to withstand high temperature.
Pictured here is when the brake fluid has been fully flushed of the old contaminated fluid. Each of the four wheels brakes have to be flushed in sequence. In the photo above, I used a one-way valve to do a complete system flushing.
If the front pads are to be replaced, I would wash the entire assembly first to clear out all the accumulated dusts. This is for my own safety reasons, as well as to prevent the dusts from entering the piston via the gaps in the rubber boot.
The brake bleeding nozzle is released while the cylinder is compressed to fit in the new brake pads. If the bleeding nozzle is not released, the brake fluid in the caliper piston will be pumped back to the master brake pump, including any contaminants, when the caliper pistons is being compressed.
If the rear brakes are of drum type, the drum must be washed to remove all brake dust accumulated that will degrade the braking performance. Pictured here is the drum soaked in soap while being scrubbed clean.
The entire assembly is also washed to remove all dirt/dust/oil, to ensure 100% performance.
This is how the rear brake assembly looks like after cleaning and dried. The drum is ready to be installed.