It would be a waste adding brake fluid when it may have too much water content. My brake fluid was clear with a slight yellowish color. So it looks ok but since I bought my car used I don't know the last time the brake fluid was changed. I was very surprised it was near 3% water which was not good. So I emptied the old fluid in the reservoir with a syringe and added fresh brake fluid.
Ditto on the use of a brake fluid tester - it also has saved me from the dealer's service department telling me that my brake fluid needed to be changed, when according to the fluid tester it was fine. As for topping off brake fluid, even though the Toyota 4.3 owner's manual states DOT3, DOT4 can be used - the difference is that DOT4 has a higher boiling point. However, DOT 5 cannot be used unless the entire system is thoroughly flushed beforehand as it is incompatible with 3 and 4.