I just redid my '51 GMC 1/2 ton completely about 2 months ago. The only parts I didn't replace in the hydraulics were those brass junction blocks (I couldn't find them either). When I tightened everything up and put fluid in it, the blocks all leaked. Oh yeah, I used preformed brake lines that were made for my truck.
After doing some research, I realized that I was using the wrong type of wrench to tighten the brake line fittings. If you don't have one, you need a flare wrench. I got one from NAPA with a 3/8" on one end and a 7/16" on the other for about $10. What a difference! The attaching nuts I thought I had tight were not. The flare wrench reaches nearly all the way around the nuts and gives you better leverage. After retightening everything, I got it down to a few leaks. I kept working with them and now my brakes are tight and leak-free.
BTW I did new (repro) Master cylinder, wheel cylinders, brake lines and flex hoses. Had about $400 in all the parts.
Good luck with it,
David Bush
