Your in-line pressure reducing valve near the rear wheel is sort of an oversized fitting. It is best to use a fitting wrench (also known as a line wrench) when removing them. Use a conventional open end wrench to confirm the size (I'm guessing about a 13 mm) before buying the tool you need. If you try to use a cheap tool or the wrong tool you will likely damage the fittings that stay behind on the other parts of the brake line. Snap-on, Mac, or Matco are preferred. Craftsman might be adequate. Harbor Freight's Pittsburg is likely will damage your fittings.
