Filling Mortar Gaps
Sweep off the brick patio with a broom, and vacuum up any loose mortar chips, using a crevice attachment. Get out any debris to have a clean, empty surface to work with.
Fill a gallon jug with water. Mix three parts masonry sand with one part cement in a bucket, wet it with the water and stir it with a trowel.
Pour concrete bonding agent into the gap where the mortar has broken away, and then fill the gap with wet cement. Use a trowel to even out the cement and scrape up any excess. Let the cement dry for 24 hours before you walk on the brick patio.
Repeat Steps 1 thorough 3 as necessary for additional gaps between bricks.
Replace Missing Bricks
Sweep the patio around the missing bricks, and sweep in the empty space where the brick was previously located. Vacuum out the empty space with a crevice tool to get up all the particles.
Combine one part cement with three parts sand in a bucket, then fill an empty jug and moisten the dry ingredients. Mix them together with a trowel until they're well-blended.
Pour bonding agent into the empty space, and then use the trowel to spread an even layer of wet cement in the gap.
Place a brick in the empty space, pressing it down into the wet cement. Use the trowel to fill the gaps around the brick with wet cement. Allow the cement to dry for a period of 24 hours before you use the brick patio again.
Repeat Steps 1 through Step 4 as needed to replace any other missing bricks in the patio.