In order to minimize the time and effort used to find the "cause" of a pump malfunction, it is best to look at the symptoms, and check the "easiest" things first, progressing to dis-assembly, if required. Parts should only be replaced if it is determined that they are the cause.
The following sections will give explanations and methods for trouble-shooting this series of pumps for the malfunctions noted. Each section will examine symptoms and indicate which things to check, so that disassembly and parts replacement are minimized.