The source has a hook up kit to use their injector pop tester for this. The kit is just a steel line with the right fittings to hook a brake line to. Injectors should be checked anyways.

Or 100.00 lands a diesel pop tester off ebay at your doorstep. You then just make an adapter up to hook it to brake lines. The adapter could just be a used line and hose fitting from a wreck. With you getting a fitting that fits the pop tester brazed on to the other end. Even new short brake lines are dirt cheap to use instead. To make up your adapter.

You always would bleed the caliper when using hydraulic force. . Before applying the real pressure. Otherwise the compressed air ahead of any fluid could result in a really serious pop out of the caliper piston. Even jamming it against any stop bar you might be using during extraction.

Mercedes dealer maintenance program for these cars. Required a check of the injectors spray pattern etc. Every 100k. There may still be a few sets of injectors in operation that are well over. I suspect I have a few of them on hand myself.