Don't worry about the pressure, but do open the filler cap. the water drain works when the engine (and lift pump) are running.
Common leaks are the hoses attached to the bottom of the filter housing and the o-ring for the fuel heater. Less common are actual leaks in the heater itself.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510