Updating to add 2 other posts I ran across. Pre-date mine, so wonder why I didn't find them. This one even used the same pump:
www.peachparts.com/shopforum/diesel-discussion/231444-easy-homemade-pop-tester.html
This one looks a bit scary in terms of not failing at 2000 psig. Always wear protective goggles.
www.peachparts.com/shopforum/diesel-discussion/348261-my-new-diesel-injector-pop-tester-1st-test.html
Please nobody freak "old post!" since that adds no value. I prefer info archived together, not the transient temporary-conversations of many posts. It also good when people "search first, ask later". Searching from Google works better than within P-P.