I recently purchased the same meter. The probe ideally should be stored in the "storage solution" at all times. I use distilled water. I am planning to store the probe in a small water bottle with a hole and slot cut into the cap to accomodate the wire.
Here's a video on calibration I got from the Milwaulee website..
My pH56 manual (very similar to your pH55) says: "NEVER USE DISTILLED OR DEIONIZED WATER
FOR STORAGE PURPOSE."
From the quick guide: "STORAGE OF THE PH55 – PH56:
Leave the protective cap off and put the unit back in a cup or jar that has 1” to 1.5” of either bottle drinking water, or 4.01 calibration solution, or MA9015 storage solution."
From the less quick, quick guide: "After use always turn the meter OFF, rinse
the electrode with water to minimize contamination
and store it with a few drops of storage
(MA9015) or pH7 (M10007) solution in the