I'd venture to say it's the poppet valves on the top of the IP, leading to the injector lines. I've heard them called o rings, but they're really look like a metal washer/disk and spring. Cheap to fix. If they leak and you have any air at all, the engine won't start since the air compresses on the injection stroke and leaks right past the valves at the end of the stroke preventing any actual fuel delivery. May also be called fuel delivery valves.