[Richard]

Yep, the inside seal on the cylinder is kaput. I haven't taken one apart, but you may be able to fix it just with a seal, instead of buying a new assembly. Be aware, it is different setup. The door is not physically attached to the cylinder rod. It is magnetically coupled.

The way those valves work, and the sister valves the ones for the dump, is that pressure is always on one side of the cylinder or the other. The pressure does not release when you release the switch. It is done that way on purpose to hold the door closed or open, and to hold the dump valve closed or open.