The Reverser/Water Pump engine blew out its top seal on me (old seal oozed down the glass). I really didn't want to take this one apart because of the smaller ~4mm sight glass tube. Interestingly, the engine also followed the Gauge engine, where as the sight glass tube slides into the upper/lower tank fittings (like the older cast iron base engines). To make things even more interesting, the tank elbow fittings had "seals" in them along with the screw caps!
I had a bugger of a time getting the sight glass out as it was really long. I've always been able, with this type of setup, to push the tube down (or up) and then slightly turn one of the tank fitting elbows to get the glass out. Not this time - I snapped off about 2 mm of glass but there was enough left to balance between the upper/lower tank elbow fittings (thank God).
Next up... what to seals for use? Luckily long ago I bought a O-Ring kit with 18 different sizes off Amazon. I used a few 3mmx1mm on each side, these went inside the tank elbow fitting and then I used a few 5mmx1mm to go into the screw top fittings. The first time I did this I got a little leak around the bottom elbow threads, so I did it all again and put teflon tape on the screw threads. Put it back together and ran the engine about 15 minutes at full steam - no leaks. Dodge a bullet on this one, that is for sure!