It comes from the ol Italians masons, the weight of the brick needs to be adressed before the mortar partialy sets up.
And the mix depends on what brick you are using and how it was made.
Look up mortar mix, and see what you get, there is plenty of info on the masonary sites.
As for leaks, that is a whole nuther set of issues. Brick absorbes water, it is not to be used without a weather membrane like 15# felt(tar)paper behind it, each seam lapped over the next.
And last but not least, there are many non professionals who do masonary work, and make the scenario a big problem.
I'm sure there are great folks who do great work out there, but they are getting scarce.
Hopefully someone with more time will explain further, and more accuratly.