I have a 97 jimmy & during the summer i installed a new heater core & fixed some vacuum lines in my dash for the heater. When replacing the lines i noticed what looked like oil on the inside of my hoses. Now went out today to fill window washer fluid & my vaccum lines that connect to vacuum ball & at the T are covered in a film of what i believe is oil. My question is where is it coming from & how can i stop it. Thanks