You can add new brake fluid to the existing fluid if the old fluid is still clear amber in color, not dark or contaminated, and you are only correcting a small drop in level. Remember, the new fluid does not “fix” old, degraded brake fluid; it just dilutes it slightly. If the fluid is dark or has been in the system for years, performing a full brake fluid flush and bleed will help you maintain your vehicle and feel confident it is safe to drive.