Help!

What's the trick to getting at the bolt that holds on the A/C expansion valve in my '86 300E? Do I need to remove the brake booster (I hope not)? Does it take a 10mm wrench or 11mm? I've tried a double ended 10/11mm box wrench that's about 8" long and I can't get enough rotation to get it to drop over the head of the bolt. Do I just need to go buy a real short handled wrench? There just doesn't seem to be any room back there to get at it with anything.

Thanks for any help.

Gary