I initially bought a Kom Cycling Sealant Injector to add sealant (Orange Seal) when needed. Then I read online what should have been obvious to me. I now also use the injector to extract the sealant to get an approximation of what’s in the tire.

I had been re-injecting the old sealant back and then adding a bit more, if needed. Lately, since the sealant is not very costly, I have discarded the old sealant and added a fresh batch, usually 1.5 - 2 ounces per tire. The whole process using the Kom injector is easy and clean.