Troubleshooting 8.1L fuel pressure loss - looking for a tool

[jocko]

Recent issue on my 01 8.1L wherein it doesn't always start on 1st try. ALWAYS starts on 2nd try, if required. Started working thru the troubleshooting tree, and have correct pressure on a fp gauge w/pump on. Once key off/pump off, pressure bleed-down exceeds specification (limit is 5 psi in 1 min, I'm seeing at least 10 psi loss in 1 min). Have verified no dripping/leaking at the pressure regulator, so suspect either a leaky injector or a bad fuel pump check valve. BUT, to isolate the issue between the two before ripping stuff apart further - the next step per the gm big book is to install the "fuel pressure test kit inline shutoff valve(s)" between the fuel rail and fuel feed (and also return) line(s).
I've got a pretty dandy fp test kit, it's based on J37287, the test kit gm calls out, but no such shutoff valves in it. Any idea where to find such an animal so I can isolate my problem a little better?

(or an alternative idea on how to test to figure out of it's the check valve or a leaky injector?)

EDIT: my only other clue is that on RARE occasion, when it doesn't start on the 1st try and I shut the key off, I hear a bit of a hollow metallic thud from the tank - like the dang gremlin is actually in there and bangs one time on the inside of the tank in an effort to get out(!) ha. My gut feeling is this issue is the check valve and maybe it's time to swap the pump out of the tank. Truck has <20k mi on it so it's had not seen much use before I bought it. Never any issues with it running perfect once it starts and it always starts on the 2nd try. SOMEtimes on the 1st, but more often than not, have to shut the key off, re-attempt start and it fires right up on 2nd try every time.