soldering problem.

I am a neophyte at soldering. I am building a manifold which consists of 3/4 copper tubing and cutoff valves. I am having trouble with one solder joint. All the rest seem to test out under pressure. It is where one of the cutoff valves connects to the 3/4 tubing. I have been heating the valve (with a torch) and trying to "flow" the solder into the joint, but it seems to keep "balling" up and will not get "sucked up" and does not make an airtight connection. I am using flux. What is the problem, and how do I solve it?

The two "secrets" to soldering are CLEAN and DRY. After that, it is a question of just hot enough. When you have had to overheat and reheat a joint, the flux has burned, and adding flux to the outside of the joint will not fix that. It will probably have to be disassembled and redone. I realize on a manifold this may not be easy.

There is a difference between fluxes, particularly in their ability to resist burnout. Based on reading here, I recently switched to Oatey #95 (the green tin that can be found at HD and most other places), and found it to be most forgiving of extra heat and other complicating factors.

A very good investment of $2 or so.

joint

Unless you remove the valve, reclean it, and reapply flux inside the joint, you will never get it to seal, or at least seal properly. There is no solder flowing into the joint and applying flux to the surface does not get it into the joint either.