Flushing an old brake system...What's a Safe chemical to use?

Gravity works. On most brake systems.
Brake cleaner is to clean/degrease external brake components, like rinsing off brake dust from backing plates.
Will remove oil from the shoes to some extent.
Denatured alcohol is one, brake fluid is the other.
considering the consequences of using anything other than brake fluid,i would stick with what actually belongs in there.