Hi and welcome to 2CarPros. Can you provide us with the exact fault code your getting when the light comes on? It seems like your mechanic is just clearing the light and sending you on your way?
The evap monitor does not always run. There are certain conditions where it will run and it will not. That is why the check engine light does not illuminate right away. Once you give us the codes we should be able to guide you.
Here is a link on evap leaks you may want to review:
Tuesday, August 27th, 2019 AT 11:38 PM