Place pork, water, and salt in a slow cooker and cook for 6 to 8 hours. After meat is cooked, remove from the slow cooker and let cool to room temperature. Shred pork and remove fat while shredding, reserving fat. (Usually, after pork is cooked and shredded, you will be left with about 3 pounds of meat.)
In a blender combine the cooled broth from the cooked pork and the leftover fat pieces. Blend and reserve for using when making tamale masa and filling. Broth can be kept, tightly covered, for 1 week in the refrigerator. The broth also freezes well and will keep for 4 to 6 months.
Heat the 6 tablespoons broth in a large skillet. Add flour and whisk for at least 4 to 5 minutes.
Add red chile sauce and salt, stir, and cook for 10 minutes. The chile sauce will be very thick at this time.
Add the 3 pounds shredded pork and stir so all the pork is well coated with the red chile sauce. Simmer for at least 10 minutes. Let mixture cool before filling tamales.
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