Brick Cleaning

We have pressured washed the large brick wall that surrounds our backyard, as well as our house. It gets the brick clean, but wondered if there's a cleaner we could spray first to lessen the amount of time the pressure washer is hitting the brick/mortar. I also want to be sure I don't apply something that will weaken the mortar. We have some black area on the brick, and those take more pressure than I like to remove.