Best way to prevent recurring leaks around chimney flashing on older roofs?

Hi everyone, I’m looking for some practical advice from roofers who’ve dealt with this kind of issue before.

On a residential property with an older asphalt shingle roof, I keep seeing recurring leaks around the chimney area, specifically near the flashing. The flashing has already been resealed once using roofing cement, and it held up for a short time, but the leak has come back after heavy rain and wind.

Before I go ahead and recommend a full flashing replacement to the homeowner, I wanted to ask: in your experience, what’s the most reliable long-term fix for this situation? Is it usually better to install new step flashing with counterflashing, or are there cases where a proper reseal and reinforcement can still be considered durable?

Also curious if anyone has seen issues where the chimney masonry itself ends up being the real source rather than the flashing.

Any insight from experienced roofers would be really appreciated.