sounds like you have a leak somewhere. i would check the fan and replace the thermostat. you keep heating it up like that you will blow a headgasket or damage the engine. not sure it a 02 sentra has a air bleed screw you have to open to bleed the air out from behind thermostat or not. is the reservoir a small bottle off to the side with just the 1 hose from the radiator going into it or a larger container with several hoses going into it? if its the small bottle it sounds like your rad cap or the hose going to reservoir is bad. the radiator is supposed to put coolanr into it as it warms up and draw it back out as it cools down, is its onlt putting it in might explain why rad was empty. also if you can park it over clean dry pavement or cardboard and run it for a while and see if it has a leak