Those leaky lower-hose drain petcocks!

[H. L. Chauvin]

Had a similar experience -- appeared to be a new leaking petcock -- installed another "new" petcock -- still leaked -- thoroughly investigated with flashlight & found pinhole leaking in joint between metal tube & female threads of petcock -- soldered pinhole -- no more leaks -- could have installed 100 "new" petcocks & "all" would have appeared to leak -- with moving light reflection, flashlights help to "see" sources of all types of water leaks.