Polybutylene [labeled “PB 2110", or “CSA-B 137.8"], is a flexible,
usually light grey 'plastic' pipe/tubing [or white, but may also be blue, black, or silver], used in residential
plumbing and hot
water heating systems, from the mid-70s through the mid-90s.
Several factors have been found to lead to the relatively fast failure of “PB” piping
"Quest" plastic fittings and crimps are one [often over-crimped], but the more serious ones are high chlorine water supply, prolonged UV exposure, and prolonged exposure to the high heat of the water, or the surrounding
environment.
The major cause of failure is simply that the
core molecular structure of the material was not well suited to the requirements of a water supply pipe, so the pipe gets brittle over time, and is accelerated by the above factors.
Once the pipe is sufficiently brittle, it only takes a small movement in the line, through sudden pressure change, being bumped, or even through temperature changes, and the line can split or crack, resulting in potentially major water damage.
This article discusses some options, for fitting replacement:
Tips for Repairing Polybutylene (PB) Plumbing Pipe
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https://www.thespruce.com/polybutylene-pipe-2718633