What I do is use the tire valve in the end cap. Attach a filler to it and blow a bit of pressure through the tubing connected to the filler and release.
Then using some dish soap in water solution I coat the surfaces that can leak. End caps, seals, etc. The slightest leak will produce and nice foam of bubbles.
I have found leaks this way when the submerging method did not or it was so small I failed to see it. And you never have to submerge it in water. All on the bench.