The fuel system of ANY old vehicle is the source of many if not the majority of problems. Several reason cause this.

The primarily cause of concern is water in the fuel system. Gasoline allows for water to accumulate in the fuel system, especially in the fuel tank. There are several reasons for this but probably the biggest is that the fuel tanks are vent and air enters the system. By its nature, air has moisture. When it enters the closed space of the fuel tank this moisture condenses over time and water accumulates in the bottom of the tank (gas is lighter than water). This leads to rust and sediment.

The secondary cause of concern is the ethanol in most of the gasoline that we buy. Ethanol is an alcohol and as such may be harmful to rubber components of the fuel system. Newer cars are built with materials that are ethanol resistant but older vehicles are not.