Deteriorating mortar is a common problem in decades-old brickwork. We suppose you could wire brush the joints to remove the loose mortar and apply a masonry sealer in hopes of stopping the deterioration, but we don't think this would be a long-term solution.
Rather, we suggest that you re-point the joints. This is the standard practice to repair worn out joints and can easily be accomplished by a do-it-yourselfer, even if he or she has limited skills.
To re-point the joints it is first necessary to remove the existing loose mortar. We like to use a narrow paint scraper - either a pointed one or one just as wide as the joint - but this is a case of whatever works best for you.
Remove at least 1/2 inch of the mortar from each joint. You may have to dig deeper if the mortar is particularly loose. This is a messy job so use drop cloths to cover the surrounding area.
Once the old mortar is removed, mix a small batch of mortar in a 1- or 2-gallon bucket. Mix small batches to ensure that the mortar does not dry before you can use it. The mortar should be mixed to the consistency of toothpaste. If it is too thin you won't be able to pack it into the joints or if it's too thick it's hard to work with.
Fill a spray bottle with water and wet the joint prior to applying the new mortar. With a small mason's trowel, fill the joints with the new mortar. Let it set up a bit, then tool it with a pointing tool. Pointing tools are metal and can either be round or rectangular. They are used to form the finished joints. Mortar, trowels and pointing tools are available at brickyards, home centers and hardware stores.
Clean off any excess mortar you may have gotten on the bricks. We've found it best to use water sparingly for this part of the cleanup because using too much water may ruin the new joints you just created. You can use a bristle brush and use strokes that are perpendicular to each mortar joint.
Once the mortar has completely dried - a few hours or the next day is fine - clean off any excess film you may have gotten on the brick with a weak solution of muriatic acid. When doing this, make sure to take all the recommended safety precautions, including wearing rubber gloves and eye protection.
This may sound a bit complicated but it's really not. Applying the mortar to the joints can be a little frustrating at first, but the learning curve is not very steep and your brick joints will look like new fairly quickly.