Update- I actually had an hour of daylight today to have a closer look, and I noticed that one of the clamps on the in-line filter was loose, so I struggled to tighten it (not easy access while laying on the ground). I then refilled the system and went got a Christmas tree.

While in the lot, I saw a puddle forming under the truck, so it's not fixed.

Got back home and had another look. It's actually the filter itself! As Confucius says "Part known new not part known good." It's a steel filter similar to a fuel filter, and the seam where the two halves join is slipping. I have all the new lines and parts for the truck, I'll just have to find the time to get it up on stands and replace them, not easy to do when it's dark by 5:00pm.