My '99 SC1 starts right up at times but mostly has to crank for several seconds

Check valve is located in the fuel pump. If you are loosing fuel pressure when the car is off, its usually either the fuel injectors leaking, or the check valve in the pump is faulty. The best way to diagnose, is to install a fuel pressure gauge and an inline "shut off" valve between the pump and the pressure gauge. This will allow you to stop return flow when the pump shuts off. If it holds pressure with the valve "off", the pump is faulty. If it continues to drop, you probably have a leaking injector. Fuel test kits that include the needed gauge and shut off adapter are usually expensive, so it may be best to take it to a shop.