Cinder block walls are just like most other types of masonry structures held together by mortar; the mortar usually gives way long before the cinder blocks do. Whether it is from moisture, heat, cold, wind, plant or animal infestations, mortar can develop cracks, lose its strength, and crumble away. When this happens, the mortar must be repointed using a procedure similar to repointing other forms of masonry.
Remove crumbling mortar by scrubbing it with a wire brush or pulling it out with your fingers. Because cinder blocks are usually held together by lines of mortar on the outside channels of the block, rather than a solid sheet of mortar, the already damaged mortar should give way easily.
Pour enough mortar into a bucket or tub for 20 to 30 minutes of work and add water, using the proportions called for by the manufacturer. Once mixed, transfer the mortar into a mortar bag.
Insert the mortar bag's nozzle into the empty joint. Squeeze the bag to push mortar out, and slowly and steadily put down a line of mortar through the empty joint. Do not attempt to fill the entire empty joint.
Scrape the surface of the wall around the joint with a trowel to remove any extra mortar. Drop that mortar back into the bag and move on to the next empty joint.
Fill any small cracks in the mortar with mortar repair compound. Press down onto the compound with the trowel to force it into the crack and scrape away the remainder.