Greetings, fellow MFers:
I believe I am the OP for the 'sight glass' thread dating back a few years. My normal disclaimer, I am not an engineer, nor do I have significant experience as a mechanic. I can, however, tell you how (for the past 2 years, at least) stopped any further pop-outs of the sight-glass while in operation.
First, I always allow a 10-15 minute warm up period before using my GC1705. Most of my usage (coming up on 800 hours since 2013) is mowing-related, or towing a trailer around the property when clearing brush, etc. My experience with sight-glass pop-outs and subsequent loss of HST fluid (at least 6 occurrences in the first 3 years)… it always happened when mowing on an uphill grade and usually within the first 10-15 minutes after engine start.
Second, after two pop-outs occurring in the same day (highly frustrating), I decided to improvise in hopes of getting my yard mowed completely for that one day. After the second pop-out, I took apart the sight glass (basically just a plastic sheath that surrounds the glass and provides a contact patch when inserted into its proper position.) I trimmed a small, thin strip of duct tape (yes, that duct tape... no rubber bands), and wrapped it along the inside edge of the plastic sheath... just enough thickness to increase the pressure of the contact patch against the metal when re-inserted. I slid the sheath (with duct tape intact) back around the sight glass and re-inserted... then re-filled the fluid.
Tapping on my wooden desk to minimize jinx risk, so far, I've had no further pop-outs. Duct-tape creativity from the hills of east TN. Gotta love it, until it doesn't work any more.
JC in Tennessee