[jmatta]

If you lost the water pump, a piece of the impeller may be stuck way up in a coolant passage and caused a crack in the head. Under driving conditions, it may be enough to allow coolant to be burned, hence the disappearing liquid. I'm certainly not an expert, but have read similar tails from Jake Raby's postings.

When I replaced my coolant 1 1/2 years ago, I used the burping method and was fine after three days of driving in various conditions.