engine won't turn over

Same batteries, (new last july), so post are in same position. I even labled them "front & back" on the sides before removing. Just removed the link cables so that all the connections were untouched. They are fully charged, checked 13.7 after removing from charger. In natural, foot on brake. All lights come on, wait till ready to start. Lights all dim when I turn the key then come back on when I release. In my manual, in trouble shooting under "engine will not crank" , cause-and-effect pressure low in tanks, correction - increase air pressure with external air source. I'm stumped!

EDIT FOR CORRECTION; Lights don't dim, what I meant was, I wait till all lights go out on dash that do and wait for the Wait To Start light goes out.