If there’s fluid in the system I’d advise testing what’s in there first. Unless you’re rebuilding the system with new seals or you can verify from previous owner what they used… because sometimes people don’t use what’s recommended.
DOT 5.1 is “compatible” with both silicone and glycol, but I can’t say if it’s suitable for racing.
A wise person once told me “Don’t ***k with brakes”