When they first came out with copper tubing and fittings back in the
late 40s to early 50s, All the fittings were cast brass. They all had
grooves cut inside around the fitting, 1/2 way down the socket, because
they believed the solder would not draw in if the fitting was tight.
There was a hole drilled in the side of the fitting so you could feed
the solder into the groove. They made them that way for about 10
years. Then they started taking old joints apart and found that the
solder sucked up inside the socket even if the fitting was tight. Of
course it is not so easy to take the wrot fittings apart, so you don't
see that. When I started plumbing, my grandfather had us young guys
clean old brass fittings, because in those days we salvaged them and
used them over if they were in good shape (he learned to do that during
WWII because it was hard to get new fittings and pipe at any price).
So I spent my spare tine taking old joints apart and saving the
fittings. The solder sucked up inside the tight ones just fine. Now
we have forgotten that. Maybe you should start buying fittings with
grooves inside them again.
Heat the tubing a little, then the fitting, then apply the solder.
Then take it apart. You will find the solder sucked inside just fine.
If you are brazing and don't heat the pipe at all, you will get a very
shallow joint. If you take the joints apart, you will see what I mean.