Making a good double flare is crucial. If you're not having any luck you may need a better flare tool kit.

Some tips are:
Be sure the tube end is square, use a file to true it up if necessary.
Debur the inside of the tube with a little countersink, just enough to make a little bevel about 1/3 the wall thickness.
With a small file file a similar bevel on the outside.
Use a drop of oil on the end.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-