For whatever reason the brakes on these rigs can trap little bubbles in them. I've bled my brakes 4-5 times, always with the ABS system and always had good luck, but when I put on new brake hoses last summer, still got a bubble in one of the rear lines after bleeding.
I ended up getting a Motive Power Bleeder and bleeding the rear brakes again. This resulted in one tiny bubble coming out of the right rear caliper. It seemed like the smoothness of the power bleeder help push it out, as opposed to the violent nature of bleeding with the ABS pump. Not sure why I had good luck so many times before, but I won't be bleeding with the ABS pump again. The power bleeder attaches to the master cylinder and uses low pressure (like 5 psi) to push fluid thru the lines.
I've also tried vacuum bleeders and don't like them....I personally don't trust their ability to remove air bubbles unless you 1) have a perfect seal on the bleeder valve and 2) generate enough vacuum to actually pull brake fluid through. In my experience, both of those things are hard to do with a vacuum bleeder.
If both you and your local mechanic bled with the ABS method, maybe you have a tiny bubble in there. If you have squishy brakes after putting on new lines but not touching anything else, there is about a 90% chance you have an air bubble in there. I'd buy the Power Bleeder and give it a shot.
Solo-bleed any vehicle with the Round Universal Power Bleeder: 2qt tank, universal adapter (1.5"–2.90") & 4ft tubing for fast, mess-free one-person service.
www.motiveproducts.com