Bilge pump, air lock
Problem to solve: Air lock in bilge pump, preventing it to pump water out.
Background: The lower bilge pump has a check-valve preventing all the water in the hose to pour back into the bilge when the switch turns off. This is a nice feature since there will be very little water left in the bilge after the pump is done. However, the check-valve (with water above it) prevents air to come out of the hose while the water level is rising in the bilge. The end result is that there is a risk for an air-bubble in the pump, that prevent it from doing it's job. It is literally "spinning air". This is an air-lock.
The solution is to drill a tiny hole in the check-valve housing, just below the valve itself. This hole will let air out while the water level is rising in the bilge and by that the pump is "primed" automatically. The hole is small 1/16" (~1.5mm), so when the pump is working a very small amount of water is re-circulated back into the bilge. /