Inspect the blue cap that is used to insert the cleaning solution. Determine if the rubber is damaged (holes, tears, etc.). Next, check the surrounding plastic area for cracks or deep scratches.

To temporarily fix cracks in the surrounding plastic area, apply a sealant like gorilla glue around the inlet of the blue cap. However, it's best to replace the whole reservoir to prevent further damage.

If damaged in any way, you can purchase a new reservoir from the iRobot website or similar sites such as Amazon or eBay.