You don't really pressure test the cat. You test if the cat is causing backpressure by reading the pressure upstream of the cat. I bought an Astro van a couple of years ago, and it had a clogged cat, found with such a test. We drilled a 1/16" hole in the downpipe and applied a manometer, and it read high. The cat was a blue blob instead of a honeycomb over about 20% of the surface. The truck went from a choking, limping, stuttering dog into a warhorse, in fact, it was a semi competent wheeler, pulled a very heavy Cobalt boat from Alcoa, TN to Gaineville, GA over the very steep Blue Ridge at Fort Mountain gap.