It has been so long I cannot remember: when screwing back in the rear piston on a rear disc a.b.s. equipt truck do you need to open the bleeder screw as not to damage the abs system?

I would suggest that to avoid backing any debris into the abs, then one of the valves sticks.

If it will not loosen without breaking your odds of causing a problem are low. If it opens no problem it is always a good idea to do it that way.