Having just worked quite a bit on my brakes and found that i only fill to half of the sightglass, i think the small amount of air below the black "bellow"seal on top of the fluid can only contain a small amount of moisture, not enough to cause a problem and need to be dried and wiped especially on the seam area to stop fluid seeping out of the lower part of the lid when mounted back in place and attacking the painted area, i also apply some grease under the lid and the screw threads to slow corrosion in those area's and provide more protection for the exposed aluminium.
The scews need to be only tight enough, if done up more they start to deform the aluminium lids, that can be checked by placing the lid on the body without the seal and white breathing plate, tightening the screws and check for a small gap between the lid and the body, maybe with feeler gauges, a piece of paper or similar.
As you have noticed before no leaks should happen in the seal area and no sealant should be used in the process, the fluid is meant to stay under the black seal and no moisture should be on top of it, the white plastic spacer has a tiny hole in the middle to let air pressure equalize as the fluid level gets lower and the pads wear down over time using some fluid from the reservoir.
Just wondering if you have also replaced the old fluid inside the slave cylinders by cleaning the dust on the pistons from the outside and then pushing the pistons back into the housing, taking the old fluid out and bleeding the line until the fresh fluid comes out after which the pistons can be released to come to their working position behind the brake pads.
A bit long winded and you may just go with popgun's advice.
Maybe some more advice is coming your way, in the meantime all the best!
