I assume it's the old style capstan head tap with a screw fit cover. It may be cross threaded in which case you may have to change the tap as you will damage the case releasing it [if it will move] I released mine using a blow lamp, not too fierce a flame and gently heating the capping. Heat can discolour the crome on the capping but by the sound of it it's the only way you'll get it off. You may have to do the same with the tap valve to get at the washer.
The best way to help prevent this is to coat the threads inside the capping with vaseline.
bob
bob, February 2005
