There is a spring inside the brake caliper piston cylinder that is supposed to push the piston and keep the pads pressed against the brake disc when the engine is not running. The caliper is semi floating like a car disc brake
There is an adjustment bolt on the aft end of the the cylinder that allows you to increase the pressure on the spring. Adjusting this, may or may not help.
If the pads are badly worn, it won't help, you need new pads.
I had this issue when in the Pacific a few years ago and I solved it by adding an adjustment bolt on the forward caliper so I could make the brake just close.
It's quite a simple system really.