workhorse 8.1 radiator cap

Thought i would update this since there were so many helpful suggestions to my dilemma. Been home about a week now. today, took off the air intake tube to get better access to the rad cap. Used a channel lock plies (had tried that before) and pushed down on the cap with a 1X3 board(had not tried before). Had to kind of wiggle the pliers as i turned it and dam if it didn't turn all the way and come off. Coolant was about a half gallon low. So i think there is a slight leak as i noticed previously.
So before next trip will drain and flush and pressure check.
Thanks again for the interest and suggestions.