I just did the wheel cylinders on my 1990 Honda Civic SI. I first removed the bolt that is easy to get at. I couldn't get that other bolt off either. So what I did is find an old junkie cheap 10mm open end wrench I didn't care about. I ground off the thick parts of the wrench on each side of the open end enough so that it allowed for more clearance, allowing enough movement to grab the bolt and turn it less than a quarter turn. Loosening it little by little while at the same time pulling the cylinder away from the backing plate. It was a slow process. The nut stayed in there, even after the old cylinder came free. Then back into the new cylinder the bolt same way, little by little until it seated. Kind of a weird design flaw if you ask me.