Finally got the part in the mail and went to remove the group valve. No proble with the first bolt (which contains the spring and the pea shaped plastic bit) but I am having a heck of a time getting the the threaded casing to which this screws into removed. This seems to require a 1/2 inch wide screwdriver (which I can't find). Is there another way to get the casing removed?
You could be lazy and try simply inserting the new pea/spring mechanism into the existing cleaned casing, but that does run the risk of there being scale on the casing part. If you are really stuck, this is an option, and you can hang on to the new casing and see how it goes.
I would normally advise using the right tool for the job, but if you can't find a substitute 'flathead' surface and the casing doesn't require too much torque to remove, you might carefully try some small needlenose pliers. Doubtful if it is wedged in, but the same trick that some folks use for their rear brake pistons.
I hesitate to suggest kludges as tools, particularly since brass is rather soft and can easily be marred by the wrong tool (bracket end/washer/dime/heavy paperclip or the arm from a binder clip used with pliers -- starting to get pretty dicey here).