Our Z's are 20+ years old. It's anyone's guess how much has leaked out over the years, or how much has been added in over the years.
Consumers and idiot mechanics add cans of freon that include stop leak and oil...how many time has this happened over the last 20 years? I cringe when I overhear the morons at autozone pedaling the "mechanic in a can" freon to unsuspecting consumers...
A full system flush allows you to start from scratch with a squeaky clean system with no contaminates.