Your pecs? That's odd. SHoulders I can see.
I'd first start doing the move with no weights, to learn to really focus in on the rhomboids. Seated, widen your back and chest, then with almost-straight arms, bring your elbows back by pinching the rhomboids around your spine. Think of pinching a pencil there so it won't drop. Make sure the movement comes from the back and not from the arm movement. YOu should be able to get a nice contraction (at least eventually) of the rhomboids this way, without any stress on the chest or shoulders. This can be a good rhomboid exercise on its own, even with no added resistance, if you really concentrate on the pinch, and hold it for a couple of counts, then release slowly.
When you've got the move isolated to that muscle group, you can add the band, still keeping all the form pointers in mind, pulling with the back and lettiing the arms just follow along.