This may be heresy but I used a screwdriver. Did the whole outside of
the brick foundation that way, gosh, like 15 years ago, still looks
The 'real' way is with a chisel like device and grinding tools if I
recall this Old House correctly (from back when they showed you how to
do things, not how to build 10 million dollar room additions).
Key is to get down to clean brick and what's left of the stable, clean,
dry mortar. The proper way also creates a shape in the existing mortar
that helps hold on to the new mortar.
But me, I'd use a flat head screwdriver, brush it out, and cram the
mortar in there.