little black box leaking coolant

[tahoe nick]

97 5.7 vortech.little round shaped box(plastic) that has a vacuum line coming outthe back running to some sort of sensor on the firewall. The underside has a plunger? That has metal arm hooked to it. Its close to the bottom of the coolant reservoir (for reference of location) its leaking antifreeze (sometimes) and my radiator,which is brand new, will not stay completely full

 

[adam74]

heater core valve? valve opens and closes to let coolant flow if you have the heat on/off. my 95 its vacuum controlled by a plunger similiar to what you would find under the dash to control doors in the duct work. sensor would be the selenoid for it that controls the vacuum.