Yes, add new fluid whenever the fluid reservoir shows fluid is needed. Additionally, change your brake fluid if the reservoir fluid is dark, murky or has significant contamination.

No, DOT 3 can be added to a DOT 4 system but only if it is absolutely necessary, for example the fluid level is critically low. After getting to a safe area it will be necessary to flush out the system and add the correct amount of new DOT 4 fluid.

No, adding fluid is a normal practice as the brakes wear. However, when changing the brake fluid the system must always be flushed first, this is the only way the system can be cleaned of contaminants. After refilling with clean fluid the system must be properly bled, otherwise the fluid level will be too low and the system may not work as needed.

Yes, DOT 4 has a higher boiling point than DOT 3 and can be used instead of DOT 3.

Yes, the color of the new brake fluid is not critical as long as the brake fluid specification matches what is recommended for your vehicle. Only the brake fluid specification is critical.

No, DOT 5 cannot be mixed with DOT 3 or DOT 4 brake fluid.

No, while the fluid level can go down due to leaks, most often the fluid level will go down as the brake pads and rotors/drums wear and the calipers must push farther to make the system work.