So I assume that that must mean that the plastic screw caps do not seal the reservoir? They are only there to keep debris out and hold the foam filters in place?
On another note, the "full" mark is literally like 1/4" off the bottom of the reservoirs. This makes it difficult to see, and the left one especially is dang near impossible to see the bottom of the reservoir. After a couple hours of mowing, expansion has raised the level by just a little, maybe another quarter inch or so, but still tricky to see. Can't I just dump oil into them so the reservoirs are like half full? That would make it easy to check with a quick glance, and since expansion is so little, there would be no risk of overflow. I mean it's just an expansion tank. I see it like the coolant jug on an automobile. If you have a little bit in the bottom it's fine, but if you "overfill" it, it won't hurt a thing. It's not like overfilling a crankcase which has no reservoir.