The reason it would run fast and not slow is beacue the fuel pump was lifting more fuel to the injector pump the. it could use so air was being flowed back to the tank.

The reason it didn't leak from the small hole was because the line was almost always under a slight low pressure air would go in fuel wouldn't come out.

Had you done a line blow back like I suggested in you would have found it by way of either the diesel.coming out or the fuel line splitting apart.