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

To add,suck out everything from master cylinder and start with fresh.
With front disc brakes,squeeze calipers in,and then remove fluid from master.gets rid of a lot of fluid. Less fresh fluid needed to flush afterwards.

a friend has a coleman stove fuel tank he uses, and flushes from bottom up. Fill tank, pump it up and away you go.