I'm not sure what you mean by "related table value" (?new terminology for coded value?), but the symptoms you describe would be expected if the coded values in the domain were not = 1, 2, and 3 with the respectively corresponding coded value descriptions 8", 10" and 12" for the valves. If so you could assign the domain to a new field with the correct field type (e.g. short integer, but could be text) and use field calculator by selecting in turn the records with 1, 2 and 3 in the old field (you could do this recalc on the original field too) and apply the new coded values .
It seems a bit odd that when you edit the persistent old values, that the pull down pick list does not appear so perhaps I have missed something or you are using a version of ArcGIS 10 with some new idiosyncracy.