Leaking roof valley strategy
Yeah, that roof's pretty beat. I'd actually go heavily at that seam with Mortar Caulk (its gritty &amp; gray), the whole length of the seam if you don't know where the leak is. But, spraying a hose at the seam &amp; very slowly moving the spray up the seam should give you a good idea of the problem area.

Still caulk the whole seam, but then do under the shingles in the leak zone. Be careful &amp; don't pry them anymore if they're brittle &amp; chunks start snapping off. I caulked a flat roof in the rain 3-years ago &amp; it still hasn't let in a drop.