The brittle tubing on Mary’s Interplak water jet continued to disintegrate, so I replaced the entire tube with Tygon:
Nisley’s First Rule of Plumbing: Never, ever look inside the pipes delivering water to your faucet.
That’s not quite inside the pipes, but it’s pretty grotendous, isn’t it?
As expected, flexible tubing doesn’t transmit the pressure pulses nearly as well as the OEM rigid tubing, so we finally bought a new Waterpik. At least you can get replacement tubing for Waterpiks, but I’ll wait until it fails before stocking up.
Contrary to what you might expect, I cut the Interplak’s cord, harvested the motor windings, and dumped the carcass in the trash.