Slacken handbrake and unclip the cable.
Does the lever return all the way to the stop.
If so, I'd suspect cables.
If not, return spring on arm or new caliper.
If external spring hooks are there, find and fit a spring.
If not, it's an internal spring.
, that is a new caliper needed.
If it is a seized cable, flooding with
, WD sometimes works but most modern cable have a plastic sheath on the inner cable. If that has picked up, it's a new cable.
As Ourkid says, maybe a red herring on the arm if the caliper isn't moving freely.
If foot brake reading are good, probably is.