Originally Posted by Jonathan R
Filter your toner and replenish it with a bit of stock solution when the activity becomes too slow for your workflow. I use paper coffee filters. If the toner seems especially cruddy, I'll filter through a paper towel first and then a coffee filter. Sometimes a bit of the finest sediment remains in suspension, but this has not adversely affected my toning.
Keep in mind that toner loses its strength gradually. Basically, you are removing some of the active ingredient with each print you tone, which means that, if you tone by timing, you should be increasing toning times slightly for each subsequent print.
I tone visually and pull the print just before it reaches a tone that I like (toning continues for a bit in the wash aid and wash). This automatically compensates for the change in toning time. When the toning time is just too slow for me, I replenish with a bit of the stock; a small amount is usually all I need (I usually just pour in a bit from the bottle instead of measuring, maybe 30ml/l to start with. If that doesn't work, I'll add a bit more).
At any rate, it's all about how the prints look for me, so I don't pay too much attention to exact dilution (since it is really only "exact" for the first print toned). Rather, I watch carefully and pull the prints when they look the way I want them to.
Your exhausted toner should come back to life by just adding a bit of stock to the solution.
Hope this helps.