Are you talking about the airbrakes in the closed position or the full open position? The only adjustment I've had to make to my 15B and another club member's 15 had to do with the airbrakes closed and locked position. In our cases one of the lower airbrake caps protruded slightly above the wing surface by about 3mm when closed and locked. In both cases adjusting the airbrake drive rod end in the wing by one half turn brought the caps flush with the wing surface just like the other ones. I experimented when doing mine and found that another half turn tighter or a turn looser and they wouldn't lock at all. We didn't need to disassemble anything to access and adjust this. We just slackened off the lock nut and removed the bolt that runs through the rod end so we could turn it as needed. The adjustment in the fuselage is is used to get both wings to lock at the same point - the manual says to check the point at which each wing locks individually and compare them. They should be within 5mm of each other. I expected that we would need to do this adjustment but the 15 checked at 4mm difference and my 15B (which had been about 2mm difference) was now less than 1mm. I checked multiple times because I had trouble believing things would work out so well. I've never thought to check how the left and right brakes compare when extended. Maybe I'll do that this weekend. My memory says that the control stop is in the fuselage and is in the part of the airbrake control system before the system splits to individual left/right subsystems.

Old gliders ARE cool when they work. Same goes for new gliders. I know a couple of people who paid a lot of money for new ships from well-respected German manufacturers and still had trouble. So far I've flown my old 15B over 800 hours and replacing the rudder cables is the most time consuming and troublesome thing I've had to do.