You are opening the possibilities of a run away engine.. Long story short you have a cav injection pump, the line you describe receives boost signal from turbo and enriches the fuel.
The internal diaphragm has failed thus it pushes fuel out the line.
This fills the turbo, turbo pushes fuel into air chamber and can lead to run away... Once this condition starts. You will be lucky to stop engine with out serious damage. Quite often a run away is a doomed engine.
This excess fuel can damage turbo bearings thus causing turbo damage.
The proper way to repair this is to have the pump overhauled, check and address turbo failure if it has occurred, remove intake manifold and empty of puddled fuel before starting.
The water pump must come off in order to remove front cover to access removal of injection pump, consider replacing wp, thermostat, timing belt and tensioner since all of that must come off any way.
Depending on your mechanic you may be looking at a 2 to 4 thousand dollar bill.
Parts alone will be around 2500 if you need turbo too.
Look into a month or so waiting on parts from the UK.
[I][/Ithe truck ran like a "raped ape"