I recently purchased a new brake proportioning valve for the rear subframe and went to thread the front to rear brake line into it. However, the thread on the brake line is larger than the recess in the valve. I threaded the other brake lines into the valve and they fitted with nil issues. Is there a part that I am missing? Any advice on how to remedy this issue?

The three fittings on the rear brake proportioning valve are all the same, and the 3 standard brake lines screw directly into the valve - no other parts, just the threaded flare nut on the brake line.

Sometimes the end of the flare nut "flares" and will not fit into the valve properly. The best solution for this is to replace the nut - not easy unless you have the tools and know what you are doing, and this can be when you find the brake line is overly rusted and should be replaced.