Water & plumbing pipes vs gravity or air pressure

Valves tend to leak, the amount depends on the design, size , seat materials, condition/age of the valve, pressure , and probably things I missed. So it would be difficult to guess the specific problem . Because of these factors , refineries use two valves in series for critical lines even though they get "good" valves and ( sometimes) verify construction quality. And well head valves are leak tight when new; however, a 3 -9/16 " ( standard size ) well head valve can weigh 5,000 lb ( for a 15,000 psi rating), ( I have no idea what the price is now).