The problem with all cranked pumps (except the six shooter) is the seal on the impeller shaft. The faster you turn the handle to fill your tank the more likely the fuel will pass this seal, due to the build up of pressure inside the pump and particularly so if you are using small bore fuel tubing in your model.

I am now using a MK electric pump which I assume is designed to work at the speed the four AA cells will provide and therefore not over pressure the pump unit. Not the fastest I have used, but it has not leaked so far, around 30 Gallons of Cool Power in 9 months, including draining the tank at the end of each flying session.