Just remembered something: the front left caliper has a tough time bleeding, it comes out real fizzy. No leaks anywhere. I thought maybe the threads for the bleeder got tapped too far and it was sucking air in there and shooting it back out through the bleeder. But maybe it’s pulling air from a piston or somewhere. A gap not big enough to leak on its own.Sounds like you’re covering it. If fluid flows when you bleed the corners it might just be a faulty caliper. Not an expert on proportioning valves but if the front end is at least diving then it sounds like the valve is doing it’s job
If the front left brake is weaker it stands to reason that the truck would dive right under hard braking … which is exactly what happens.