Tandem Pump has a pressure relief valve?

In the process of partially rebuilding my 2006 Jetta I got into cleaning up the tandem pump and stupidly removed the hex drive plug for what i am guessing looks like a pressure relief spring and seal. I can't find any reference info about this item and what worries me is how do you replace the plug at the right depth? The spring pressure is very sensitive to where the plug is. Is there a way to measure this?
Grateful for any advice!