A moderator can move this post to the main message board, you will get more exposure there.
But I'll answer what I can in the meantime and there are lots of other members that know more than I do. The rear port on the prop valve serves the rear brakes. The line coming from the prop valve for the front brakes should be 3/16" and it will run down to the tee and split into (2) 3/16" lines. Are you using a prop valve meant for a disc/drum combination? If so, you should just have one line from the prop valve for the front and one line for the rear.
Here's a link to a thread with a picture and some info:
http://67-72chevytrucks.com/vboard/s...d.php?t=640142
If you don't have the right parts, I would call LMC and have them walk you through it. They're most likely Inline Tube brake lines. I got a full brake line set from GMCPauls (distributor for Inline Tube) and they sent the wrong front to rear line, so it's not uncommon for the wrong lines to be sent out.