If the system held 16 PSI with the pressure gauge on it as it's supposed to, check and replace the radiator cap. There's no way for coolant to escape from a system that is fully sealed everywhere else but the radiator cap, which is where you hook up the pressure tester. 

Try the test yourself and rent a pressure tester kit, raise the pressure to 16 PSI and let it sit. It shouldn't drop off much. Check it after about a half hour, if it's dropped significantly, something is seeping coolant. On a car that old, it could be anything in the cooling system. Suspect the water pump (check the weep hole), all hoses, possibly the radiator and heater core as well.