Some time in the 1980/90s Ford changed to a "different" style of cooling system. Actually, they revived a very old style of cooling system. One where the reservoir for the cooling system was remotely mount from the radiator (on a typical radiator, the very top part of the radiator is actually the reservoir).
With this type of a system, the remote reservoir is under the same pressure as the rest of the cooling system. When this type of system was first introduced in the 1950s the tank was made out of brass or copper. When Ford brought it back the reservoir was had out of heavy, high temperature plastic, but it is still pressurized.
This system works very well and has advantages over a traditional cooling system with an overflow tank. The biggest advantage is that "burping" the system (expelling air) is no longer required. When filling an empty system, fill through the remote reservoir. Run the engine up to operating temperature, shutdown and let fully cool. On more top off and you should be good !
Bottom line, if that tank develops a leak, buy a new one from your Ford dealer. Don't trust aftermarket tanks !
