I've been asked this twice in a week, so here's a reminder of classic E38 tricks that may apply to other boats.
If an air lock develops in fresh water plumbing (no pressure water), then run the water pump and:
1. Seal end of the sink overflow vent for a moment with a finger.
2. Pump the foot pumps.
The main tank vent runs through the foot pumps. Since they don't get used a lot they dry out and develop harmless air leaks. Pumping them reprimes the system.
If an air lock develops in fresh water plumbing (no pressure water), then run the water pump and:
1. Seal end of the sink overflow vent for a moment with a finger.
2. Pump the foot pumps.
The main tank vent runs through the foot pumps. Since they don't get used a lot they dry out and develop harmless air leaks. Pumping them reprimes the system.