emjay
+1y
I think that I read somewhere that these porp valves (or any for that matter) can go bad if there is a slight restriction anywhere in the system. Try taking it out and giving it a good cleaning, then reinstall, bleed, etc etc etc. I don't think that only one brake would seize up if the whole valve were off though, but I'm by no means a brake expert.
EDIT: found this on another forum where a guy thought that it was his porp valve. Turns out it was something else. makes for a good read: