There are two reasons why you should regularly check your sealant:
- Sealant drying up
- Sealant getting old
You should always have the amount recommended by the sealant manufacturer in your tire. You will develop a feeling, over time, of how frequently you have to check your sealant. It’s always good check your sealant before a long ride or a holiday to have the peace of mind that your tubeless system will be all right.
Sealants also get old and lose their functionality. Old sealant can be recognized by a change of color or when it starts dissolving and the consistency is more like water than like new sealant. Old sealant should be extracted from the tire with the syringe and disposed of. Mixing new with old sealant is not recommended.
milKit sealant does not dissolve and just gets thicker over time. Still the sealant has to be replaced regularly to guarantee the best performance.
More about the new milKit sealant can be found here.