Sounds like you may have 2 diff problems.. First, Your Battery and brake light flashing.. Thats usually a charging system issue , meaning Alternator most likely is starting to fail. I would have your Charging system and Battery Checked. 2nd your temp issue. Depending upon mileage and Service would help me point out further diagnose. such as Servicing Coolant? timing belt? Water Pump , OR thermostat. It could be a simple air pocket from a coolant service. Additional info would be helpfully on the 2nd