In 3-quart saucepan, combine vinegar with water, salt, sugar and mustard seed. Over high heat, heat this marinade to boiling. Reduce heat to low; simmer 5 minutes; cool until warm.

In large bowl, combine cabbage, green pepper, pimentos, onion; pour marinade over vegetables, then toss to coat well.

Refrigerate relish, covered, 2 hours, tossing occasionally. Toss again before serving. Eat chilled, with cold sliced ham or pork.