Resealing the Single is not really hard, however the first part is usually the hardest, namely getting the valve out of the body.
Unless you have a Hammerli valve removal tool you will need to make one. A length of flat bar just under the width of the diameter of the cylinder is required and long enough to allow some good purchase. The thickness needs to be enough to slot into the cut outs in the valve. Firstly remove the CO2 capsule return spring with a dental pick or similar. Then insert the tool into the slots in the valve and turn anti clockwise to undo. If your lucky it will come undone, if not you may need to soak the top of the old seals by pouring or spraying some oil down the tube and leave to soak for a few hours.

Once its loose and undone as far as it will go, you need a length of brass to put down the barrel and push the valve out. I use an old brass cleaning road.

Hopefully you will have a steel bodied valve like this

[IMG]MVC00022 by Mark Davis, on Flickr[/IMG]

If you do, then you may only need to replace the two main seals which are 13x3mm OD and need to be CO2 proof. A light dab of silicone grease is all you need on them. If the seal on the tip of the valve is in good condition then leave it, if not you will need to get a new one.

If you have an aluminium valve you will have a 'top hat' valve which will need a new polyurethane seal in order to make it gas tight.

Exploded view of an early model:

[IMG]` by Mark Davis, on Flickr[/IMG]

Let us know how you get on