I had a mystery leak into a 1st floor bay window only during wind driven rains. After thinking it was a flashing issue above the bay window I had the counter flashing replaced. Leaks continued only during wind driven rains. My culprits were the window sills of two windows on 2nd floor above the bay.
-I ended up having all seven rowlock sills replaced with limestone sills on front of house.
At first my worst thought was that the brick veneer was screwed up somehow. I had a local company that does brick restoration come in and he showed be a Rhilem Tube test to demonstrate how the brick can absorb water.
Here is an excellent video demonstrating a Rhilem Tube:
I know in your situation you do not have the window sills as I did but maybe this can assist.
Water repellent vs. Brick sealer is a big big huge difference. I would not take the advice of applying any type of sealer on the brick. That may trap any moisture behind the brick and cause further problems. What you want to do is apply a water repellent...not a sealer.
After learning that brick veneer--mortar joints and bricks--can absorb water, I had a water repellent made by Prosoco
called Siloxane WB concentrate applied by to entire front brick veneer of the house. After the application, water beads off of the brick veneer as water woul off the hood of a newly waxed car.
product will not impair the natural breathing characteristics of treated surfaces.
As a note to others reading this. Above I referenced the Siloxane WB concentrate. This needs to be mixed on site with specific quantities of water. The company that applied it for me were the experts so thats what they used. They did tell me if I were to do it myself they recommended the Siloxane PD which is pre-diluted. Its ready to be applied right out of the container.
But because the repellent will not prevent water penetration through
structural cracks, defects or open joints I also spent a few hours paying attention to all the mortar joints....both head and bed where there were any cracks in the mortar. So I bought and brushed on a product from Saver Systems called ChimneyRx® Masonry Crack and Joint Sealant.
look for hairline cracks in all the head and bed joints...see photos below