Its not hard to get the moisture/ fuel out of the mercury... I recently did this and considering the density of water or fuel is much less than mercury (hense the fact that it floats on top of it) I just used a syringe to pump out the mercury back into the bottle it came in. Just suck out the mercury from the bottom of the reseviour until you suck up all the mercury leaving the moisture/ fuel bubbles behind. Then I took the tubes, blew them out with an air compressor, and hung it up and let it evaporate. Now when I need it again, I'll just pour it back in and do it all again.
The only thing you are trying to get at is all 4 carbs pulling the same level, it may jump around a bit but just keep tuning it at differnt rpm ranges, basically the higher the rpms are the more stable readings you will get, just make sure not to suck the mercury up to high and into the engine. I wouldnt bother making a new setup, you already have one that works.
Brian