You could try draining the cooling system and remove the bolts one at a time and apply Number 2 Permatex which is a black sealer that you put on the treads of the bolt and reinstall them one at a time and torque them down. This sealer is non-harding so it can expand & contract with the heat of the engine. Do not put a silicone sealer on the bolts it will set-up and the coolant will still wick around the bolts so the leak will just come back over time.