How should I repair crumbling joist pockets in a brick wall?
You remove the old brick and rebuild the bottom of the mortise.

If it was me, I would get a tile saw and cut a single rock that would exactly fit. Why use crappy brick, when you can use a rock that will last for a million years? If you cut the rock so that it exactly fits (and I mean EXACTLY to a hundredth of inch). Then you hammer the rock in tight and you need no mortar.

If you don't want to spring for a tile saw, you can use cold chisels.