Brian got this email back from the manufacturer when he inquired as to replacement parts.
The Waterpik® oral irrigator model WP-20 was discontinued in 1999. Parts are no longer available to support this product and specifications are not listed. We recommend replacing the machine.
Needless to say he will not be buying anything from Waterpik®.
We should buy from companies that repair and supply replacement parts for a number of years after the product stops production.
Rick McCharles 
Yeah, I recently had a problem with my WP-20 as well. The hose just broke during use one day-snapped, fell apart. Needless to say, I was shocked. Anyway, I got the same response from Waterpik, and I was pretty steamed, especially since everything else to do with the pik works just fine. However, there is some help available-depending on what’s broken. For example, there’s replacement tubing available, that I plan on trying. Also, I’ve seen online that there are also replacement reservoirs as well as reservoir valves available as well. But you’re right, they should still be making replacement parts available for this model. I guess the only other option is trying to score a used machine somewhere and just cannabalize it for parts 😀
Along the same line, no one supplies a replacement reservoir valve for any of their units. I would not buy another of the products