Hello Lewis,
The reserve tap is actuated by a thin flexible wire, not a cable as in the case of the choke. Given that when you push the reserve knob a resistance is felt and then removal of your hand sees the knob return to the open position would suggest something is either corroded or damaged.
It may be advisable to seek the assistance of another person so they may watch the reserve tap while you push the knob in and see if any movement is occuring at the tap end.
The tap itself is all brass and contains just a rubber "O" ring. You can replace this with another petrol resistant "O" ring should it be found necessary. I am more inclined to think that the actual casing that houses the wire may be corroded and is cause.
You can also detach the wire from the reserve tap and try pushing in the knob and see if that makes a difference. If so, then the problem is in the tap, if not then the casing and wire will require attention.
Ron.
