I most certainly does change when you add in the halide!

Look up the Ksp of AgBr, and then look up the definition of Ksp (solubilty product constant - the Ksp is the product of the ions in solution). Try the math now with that added into the solution. The pAg will swing back into higher numbers (i.e. lower concentrations). Ammonia will mess with it a bit as it will increase the solubility of the silver by forming complexes.