Cracking or spalling in bricks may occur when the wrong type of mortar is used. There are no easy solutions; you may need to either replace all the mortar or rebuild the wall.
If the bricks in your siding or brick wall are breaking apart, simple patching probably won't work. Any patch material you apply will just pop out again unless you solve the underlying expansion/contraction problem. The bricks in this photo, which are recycled Chicago-commons (a soft brick used at the turn of the last century) are cracking because they were set with one of the newer, nonflexing Portland cement mortars. The bricks move slightly with changes in weather, but the mortar doesn't.
If you have a problem like this, there are two choices. Either reuse the old bricks but with the right mortar (lime mortar or lime putty), or rebuild with new bricks. Pick a brick that's harder and more weather resistant.