it does make good sense that the leak would be in the mortar sections. Mortar is porous and can actually have holes the size to allow water to weep in.
I would definately consider the silicon sealant for the bricks and mortar. This would work just like sealing the grout of ceramic tile.
The solution can be sprayed on also.
But, the down side, it would have to periodicly be re done.
The City of Corpus Christi sprays all the bridges and under passes with this chemical to cut down on mold, and stains.
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