The washers were missing and I couldn't get a grip on them with vice grips or channel locks. I don't have a grinder or cut off tool here. So I used capillary lube on the threads at the brackets, then drilled them out until I could hammer in a small hex bit. Came out easily after all that!

Should I replace with steel or brass M8s? I can't tell if they're a security or safety (shear in a crash) feature.