The fill line on the probe cup is the minimum volume for tested sample in the probe cap, not for the storage solution. The BPB conductivity sensor should not meet KCl solution. The BPB sensor can be stored dry. Only the pH sensor needs to be soaked. 

So a good practice to soak the PC60 multi-parameter sensor is to pour in a little bit KCL storage solution in the probe cap (about one-fifth of the cap) that’s just enough to submerge the pH glass bulb sensor. Soaking it for 5-10 minutes will recover the pH glass membrane’s sensitivity pretty well. To achieve highest accuracy of the pH sensor, soak it overnight.