On old brick work, it may actually be a requirement. The mortar may need re-pointing, and even then you may still have a problem with old porous brick. It is becoming more common here in Texas to need and use brick sealers. Most of our brick isn't nearly as old as much or yours.
And I second the advice to check the crowns. We just replaced two crowns on two big chimneys. One was starting to crack apart, and the other had long ago collapsed down into the void between the flue and the chimney facade. This house probably cost a half-million to build and they used plywood-under-mortar for the crowns - go figure.