I have a similar problem and just had a chimney expert out on recommendation from my roofer. We have repointed, capped, sealed (with silicone sealer) and put a cricket flashing up. The problem still exists. My chimney guy suggests a sealer that is NOT silicone based. I had this done about three years ago and it has worn off. It was expensive, didn't last long, and doesn't allow the bricks to breathe creating more of a problem. This chimney expert suggests two things that make sense. 1) to seal the chimney with a product called Chimney Saver. There are many videos on youtube of people using this product. It allows water to escape the brick but doesn't allow water to penetrate the brick from the outside, allowing the brick to breathe. 2) My water heater is vented through a flue that he found to be blocked. This creates condensations, often at the roof line. If seems as though when it rains the wall appears wet but apparently it's all about the air pressure and temperature outside that can make this happen inside which often changes when it is raining. He is installing a metal flue and properly venting the water heater. This will keep any condensation from the water heater away from the brick and it will drip down the metal flue and catch in a T shaped tray at the bottom and evaporate. Everything that he said makes sense to me!