Many older pools have copper plumbing which was the Industry standard up until the mid 1970s. The standard now is scheduled 40 PVC pipe. The problem is Copper pipe wears out as water travels through it, eventually wearing the pipes thin which in turn leads to leaks.
Copper also leaves a blue/green residue on the surface of the pool making the Plaster look old and unattractive.
If your pool is over 40 years old, Re-plumbing is definitely an option to consider.