1970-71 trucks use a case storage system in lieu of the charcoal canister. This system's basic function is to prevent the escape of fuel vapors by routing them into a storage case for later combustion. The only scheduled maintenance for this system is the replacement of the system's air filter every 12,000 miles (20,000 km). The filter is located above the fuel vapor storage case, next to the fuel filler cap. Unplug the old filter from the hose, discard it, and insert the new filter.