Water that old chestnut with Live Steam. Use of inapropriate water could indeed mess up sight glass.I often wonder if the water is to blame with regards to the number of problems experienced with sight gauges. I have never seen any water treatment offered that might alter the surface tension or scaling of the different waters - is there such a thing for model steamers? Or do you just use water produced by condensation?
Another small tip with sight glass is to do a Diagonal Set of lines to the rear, I use those Chalk Felt Tips for this. The line then magically straightens where the water is but not where it isn't. All to do with refraction or something I believe.
JonD