After the engine warms up the temp gauge reads hot then levels off and will stay level while driving until i go uphill, then hot and levels off. the water and coolant in the resevoir disappears and needs to be refilled after 20 miles. i have replaced the heater core hoses and thermostat... the radiator hoses appear fine and there is no visible leaks from cooling system. i cant find the valve to bleed the system of air... and not sure if i need to replace the water pump at this point..or is it the heater core? the heater does not blow hot air either? wtf