Ok, bear with me as I try to explain this. I've got a 98 XLT with a full length bed (if that matters). I have a brake line leak in the rear, above a junction block (proportioning valve?) that is mounted on the rear axle housing. Here's the part I don't fully understand:

The main line comes into this rear block (presumably from the proportioning valve in the front somewhere). Two hard lines go left and right to feed the two brake cylinders. There is a 4th line, that goes upward into a sheet metal cross member on the underside of the bed. This is the leaking line - and it appears to be leaking right at the bed or inside the cross member.

Can anyone tell me what this line does? The only thing I can think of is that it's a pressure relief line of some sort. Regardless, how difficult is it to replace?

(As usual, my Haynes and Chilton manuals were zero help with this)

Thanks,
dogfood