It's NOT a problem resulting from using DOT 4 fluid. DOT 3 and DOT 4 are made from the same thing - is a polyethylene glycol-based fluid. The DOT 4 rating merely means that the fluid meets a higher specification for minimum boiling point: Dry boiling point Wet boiling point DOT 3 205°C (401°F) 140°C (284°F) DOT 4 230°C (446°F) 155°C (311°F)