In a medium saucepan, combine water and kombu; bring to simmer over medium heat. Remove from heat and stir in bonito flakes; let stand 5 minutes. Using a fine mesh strainer, strain out and discard solids (or reserve for a second use). Return liquid (dashi) to saucepan and add remaining ingredients (pork broth, chicken broth, soy sauce, rice vinegar and mirin). Bring to simmer. Serve immediately (see post for assembling a shoyu ramen bowl) or cool and refrigerate for up to 1 week or freeze for up to 1 month.