the non worn piston is a little stuck.

remove pads

depress brake pedal several times whilst watching pistons come out

clean with brake fluid and toothbrush.

push back in

test.

if this fails, you need a seal / piston kit

__________________
C140 420 CL
W114 280CE pimpmobile
W114 250/8 Hillclimber with M110 and bike carbs - got any hot cams?