I've read this with great interest. Seems you have touched on everything possible. My two cents. I would be leaning towards a weak brake sring. It doesn't pull on a normal stop only on a hard stop, right? It just seems to me one of the strings could be weaker than the rest. Just a thought. Someone else has mentioned this so I tend to agree with them. I will keep u with this and see what happens. I am almost in the process of replacing all the brake lines on my 49 1/2 ton and also the master cylinder. Hey, at least you have brakes. I have none and can't get them to bleed. Oh well, I'll get it. Good luck.
