What is the trick to making leak free welds?

[Reverser]

The tiniest bit of moisture, oil or back pressure can cause a pinhole. I welded a crack on a ripper frame on a cat and while it was cooling cracked open again because of the pressure build up inside the tubing.

I had a similar problem years ago while working on a RR freight car that had been sent in to the shop for repair.
A small short air brake line had broken off a welded flange that attached between two air brake valves. It was hard to get to, but I crawled in among the mess of pipes and other things in the way, and pushed the pipe back into place and welded it.
I hooked the air up and could hear a small leak. After soaping the area, I daubed the pinhole leak with more weld and cut the air back in. But, it still had a pinhole leak.

After 2 or 3 times of doing this, I was frustrated and mad and took the pipe completely off and put it on the table and welded it again. This time it was fixed, but I was always puzzled as to why it kept leaking every time I welded the pinhole while it was still on the car. I finally figured out that the valves on either side of the pipe had the air trapped in the pipe and must be causing a pinhole as the heated air was forcing its way out through the weld puddle.

 

[Arc weld]

Just think of all those welded pipeline joints that don't leak!

Course the welder has had courses, lots of practice and yearly competence checks.

There's a big difference between welding pipelines and welding hydraulic tubing. Hydraulic tubing is usually silver soldered and/or uses flared fittings.