If you undo your new line that you teed into the return rail and either let it dump into a bucket orrrr make a mess and let it drain to the ground if the engine still runs on then it is as Jim said...probably an injector check ball or maybe a centre oring gone on an injector.
You can isolate the bad injector by holding the injector rocker lever fully down so the plunger is bottomed in the cup while someone else turns the engine off.....do it for each injector.....the bad one should show up by allowing the engine to stop straight away.