Hanging valves are mostly caused by moisture finding it's way into the guide and stem area and eventually causing damage to both.
The moisture is mostly caused by short-trips, and this same moisture will play hard on non-stainless exhaust systems as well.
It's for this reason when we do any Flathead rebuilds, whether stock or hi-perf deals, we always install bronze liners in the guides. Since we've been lining the guides we haven't had a single valve "hang". Not a one!!
It also helps to use some decent "Stainless" valves, this helps big time.
I know it's more difficult for a "fix" when it's a running unit, but it will be a constant battle in the end without the liners.
Thanks, Gary in N.Y.
P.S. Most customers really don't care for the add'l initial cost to do the liners on an overhaul, but you can see the aggravation it saves in the end. No need to go back inside the build, saves much labor and expense (gaskets, etc) to do it right the first time. Every Flathead we do gets the guides done, the "Stainless" valves, and Viton stem seals. This is a non-negotiable item here on new builds. Here's a shot of a finished guide with the Viton seal.