Ok, found the solution. You need to pass 'nil' for the unchecked_value
Incomplete - <%= f.check_box :sub_status_fk_equals_any, {:name =>
"search[sub_status_fk_equals_any][]"}, "51", nil %>
Complete - <%= f.check_box :sub_status_fk_equals_any, {:name => "search[sub_status_fk_equals_any][]"}, "52", nil %>
Under Review - <%= f.check_box :sub_status_fk_equals_any, {:name => "search[sub_status_fk_equals_any][]"}, "53", nil %>
Approved - <%= f.check_box :sub_status_fk_equals_any, {:name => "search[sub_status_fk_equals_any][]"}, "54", nil %>
Hope this can be of help to someone else