When we finally opened up the wall, we discovered that the through-wall flashing had been pieced in, with the seams simply lapped, not sealed. Water was draining off the housewrap and onto the head flashing, but it was leaking through the seams. Plus, there were no end dams, so water was spilling off each end of the flashing and finding its way past the housewrap, past a layer of foil-faced polyiso insulation, and into the wall.
The thicker material stands up to abrasion from masonry and tends to come in wider widths, so we can get a good lap behind the housewrap and still bring the flashing all the way out to the face of the brick. This last point is important because it prevents water from soaking back into the masonry below the flashing and finding its way into the framing. There are purpose-made flashing materials with metal drip edges, but they are expensive, difficult to work with, and aren't stocked in our area. On some jobs, we have fabricated stainless steel drips along the outer visible edge.
Most of the time, however, we just have the masons bring the flexible through-wall flashing all the way out to the face of the brick.
The other critical detail on any through-wall flashing is the end dam, which is simply a fold at each end of the flashing that brings the material up into a vertical head joint. This detail allows the flashing to function more like a collection channel, or spout, than a flat membrane. Creating an end dam is also more reliable than trying to seal overlapping pieces of flashing: Whenever we need a mid-span joint in flashing for a long run, we create end dams, turning the ends of each piece up into a head joint.
We are careful to flash around windows, using high-quality butyl-rubber flashing and properly integrating it with the housewrap and window flashing.
A key feature of a good window flashing is a drainable sill pan, which gathers the water leaking around a window unit and directs it to the outside. We find that a lot of masons, if left to their own devices, will let the sill pan drain into the air space behind the brick, leaving the through-wall flashing at the base of the wall to do all the work. However, we feel that best-practice should include a through-wall flashing right below each window that is integrated with the sill pan. Any water leaking from any window needs a clear path out of the wall as quickly as possible.