A leaking A-key will make the instrument unplayable.
First, check with a piece of cigarette paper, which you place under the A pad, if you have a firm grip. If not, take off the G# key and then the A key, and make sure that there is no corrosion on the A spring as well as the little spring which is mounted inside the A spring groove.
Sometimes this spring might also be pushed out of its place. In this case push it back.
Sometimes the A spring might be pushed toward one side of the groove, which disables the A key to close either. Try to place it back into the middle. If it tends to get back to the side position, remove any bure in the grove that might cause it. Bend the tip of the spring so that it is round and can glide on the surface easily. Make sure that the spring is bent symetrically.
Another problem can occur, when the A-spring ground spring is pushed out of its origianal position. It should never stick anywhere into the tonehole area. It must always lay safe and straight in the groove.