I guess you mean it's a modern one that overflows into the pan rather than through a pipe to outside.
Either the inlet valve sticks open, so it's always overflowing, or the syphon is sticking. The syphon could be staying partially open.
If it's the inlet valve, it's often a piece of dirt keeping the valve open, but if releasing the pressure fixes it, then it's probably just sticky.
Replacements are easy to get hold of anywhere. It shouldn't matter about make and model etc. They're universal.