Hi '71.., welcome..!!
A bit more info. 'bout your rig would help so as to get correct help.
Filling in your sig. page with any/all you know will show on every post & can be updated. (You likely have a D-30 unless someone changed it to D-44)
In the upper right on this homepage (search box)., type in disc brakes., power disc brakes., power brake booster, etc. You will see lots of info.
If your truck is lifted, those lines (at the difs.) may be longer than stock.
If any lines, at the wheels, or dif. centers, are cracked/aged they should be replaced.
---(I can't tell you if new lines now, will match up to new disc's later)
---However., the flex lines attach to a "hard line" on the knuckle which in turn attaches to the brake system. So my guess would be that your "new" lines now would not have to be changed for disc conversion.
---(Some folks build their own mix-n-match disc brake kits.)
However, the EB stores now have complete kits with ALL new parts., including lines, mast. cyl., proportioning valve, etc. (D-30 & D-44)
Generally, disc's & power boosters go together for proper performance.
Check out Tomsbroncoparts.com ..., &/or Wildhorses4x4.com, if you're on the left coast. Everything you'll need is there.
