The fill line on the probe cup is the minimum volume for test sample solution in the probe cap, not for the storage solution. We wouldn't recommend storing the BPB conductivity sensor in 3M KCL solution as it could cause some problems to the black rods. The BPB sensor can be stored dry with no problem. Only the pH sensor needs to be soaked in the 3M KCL solution if it has not been used in a long time (> 1 month).
It is a good practice to soak the PC60 multi-parameter probe by pouring in a little bit of 3M KCL storage solution in the probe cap (about one-fifth of the cap) that’s just enough to submerge the pH glass bulb sensor. And let it stand in the hole of the carrying case. Soaking it for 20 minutes will recover the pH glass membrane’s sensitivity pretty well. To achieve highest accuracy of the pH sensor, soak it overnight.