A couple of months ago I purchased a Pentax 67 system that came with an old 6x7 body dedicated to a Polaroid NPC back mainly as a throw in. I got a bit excited and decided to start shooting Fuji Pack film for fun. Unfortunately the 6x7 body did not fire even when the battery check was lit with the red light.
I assumed the camera was broken and got rid of it for parts on Ebay but kept the NPC back.
A week ago I purchased another 6x7 body from KEH and have the same issues. I remember reading somewhere that the older bodies needed film in order to fire. Is that true? If true, how do you shoot with the Polaroid back?
Any help would be appreciate it!

Thanks!