Chimneys can leak for a variety of reasons, not all because of deterioration in the condition of the chimney.
Masonry chimneys are essentially porous, so given enough rain (like recently in Portland), any chimney can leak.
If a chimney is leaking we take a systematic approach. Any repairs should be addressed, cracked or worn mortar joints, loose bricks, etc., should be repaired. Rain covers should be installed, keeping the water out of the inside of the chimney (the flue). A custom cap covers the whole top of the chimney, preventing seepage from the top down. Usually chimneys deteriorate from the top down, due to water penetration and subsequent freeze damage. When wet chimneys freeze, they crack, allowing more and more water to enter.
Water and freeze damage over time (or suddenly if there is a freeze after a period of heavy rain) is a primary reason we repair chimneys.
We like to waterproof each chimney we work on with a water repellent that lasts 10 years. If water can’t penetrate and freeze the chimney should be in pretty much the same condition in 10 years.
A water repellent is preferable to a compound that puts a “skin” (like paint) over the surface as it allows the chimney to breathe, not allowing moisture to be trapped where it can freeze.
Chimneys have to be pretty dry when sealed.
We also wrap chimneys with plastic or shrink wrap temporarily until repairs or waterproofing can be done.
In general, if a chimney is in good repair, flashings are installed correctly, is waterproofed and has rain caps, it will not leak even in the worst weather.