The brake fluid reservoir is usually plastic and clear or ... to test the fluid myself and see how contaminated it was. The type of test I used indicates how much copper is in the system, which ...

but like brake calipers, sometimes it can spring leaks or otherwise fail, and it must be rebuilt or replaced. The master cylinder is topped by the brake-fluid reservoir, which must be kept full.

Clean any dirt or grime off the master cylinder reservoir cap with ... Yes, but never mix different types of brake fluids.