[Mark IV]

The original applications for these tanks in Ford "FE" powered vehicles are the reason so many of these tanks fail. For example, in the 1964-66 Thunderbird 390 (and 66 428s) the factory installed upper hose is known as the "question mark" hose i.e. the shape approximates a question mark. This allows the engine to move and the hose to flex and take up the movement. The aftermarket replacement upper hoses are general just a 90 degree shape and thus "pull" on the tank when the engine moves. Most commonly the solder joining the bracket to the lower tank body fails and then the tank is pulled every time the engine rocks. As this continues the joint holding the upper tank body to the lower fails leaks ensue.

The Kirkham tank, the Southern Automotive tank and the polished stainless steel tank that is optional from Superformance (and for the Shelbys they build) do not suffer from this malady as they are not soldered and are much more robust.