Is the radiator loosing any coolant? Do you see any coolant leaks? If it is just unedr the resevoir than maybe the resevoir is all that is leaking and has a hole in it or the hose on bottom came off or is cracked.
You may need a pressure tester and pump the system up to about 15lbs and look for leaks. Usually most auto-parts stores will allow you to borrow there tester as long as you dont leave the parking lot with it.
Dont forget to check the cap, too.
It might be a good idea if the engine already has alot of coolant all over it to take it to a carwash and use the sprayer and clean the engine.
Is the any air bubbles coming up in the radiator when the engine is hot and running, indicating maybe a blown head gasket? Any coolant coming out the tailpipe when running? Any coolant on the engine oil dipstick? Any coolant on the spark plugs?
Look underneath behind the water pump, there is a small hole that usually when the water pump goes bad it leaks from that small hole. Look for green stains all around the radiator core and the end tanks and around the hoses. Also, up around the heater hoses where they go into the firewall for the heater core. Look around the thermostat gasket and intake gasket for green coolant leaks. Also, all of the freeze plugs. Make sure the petcock is tight. Look around the head gasket, too. Plus around the coolant sensor.