I'm renovating an old row house with wooden joist in masonry pockets. In many places, the mortar under the bearing point of the pockets has crumbled, cracking a few bricks and lowering the pocket. Assuming I can replace or at least temporarily move the joists, how should I repair the bottom of the pocket?
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.