After I replaced my split radiator (S5 NA) and got all the hoses tight enough to stop leaks I still couldn't keep it full. The funny thing was no matter how much I drove it, the next morning it would be 1 cup of fluid low! A quik trip around the block or an hour of driving, always 1 cup low. I changed radiator caps, overflow hoses and clamps, looked everywhere for a leak, and added a can of stop leak. No change. Finally, I realized my overflow cap was broken and not sealing solidly onto the plastic bottom tank. I didn't think that could be a problem since the bottle I thought was open to the atmosphere anyway. I found the same cap on a 323 at the junk yard and swapped it on. After two days, no loss of fluid! So, must be the overflow operates at a slight positive pressure maybe?