It's no big deal to replace the valves the only thing you want to make sure of is that the tanks are empty.
There are two ways to stop the valves from leaking and that is when you take the valves off you will be able to see the area where the valve slices into it's grove now you can eather spray some silicone on the valve shaft and on the inside of the cutter like stopper.
You may be able to spray inside with out removing the valve.
The other options is to replace the valves.
Thats what I had to do after a while.
Just a thought.