Here's some valuable tips:
When drilling the holes, angle your drill 1-2 degrees upward i.e. drill slightly down into the brick. When mounting the shelf, the natural weight of the shelf will compensate and result in a perfectly flush floating shelf.
When mounting the T-bracket wall plates, ensure that the 2 holes on the wall plate are positioned at the top of the wall plate (not the bottom). This stabilizes the wall plate against the wall.
Do not put the wall plug into the hole first, screw the nail-in anchor into the wall plug and then slide them both through the T-bracket holes, into the holes drilled in the wall. Not doing this could result in the shelf being semi-flush with the wall.