Hi James, I have gone threw your brake trouble and notice that nothing has been said about the equalizer, do you think that it might be slightly stuck to one side? This could cause a turbulence in the fluid flow. My 420 had a brake issue and it turned out to be a piston that would wedge and after some use you could feel a sudden pop in the peddle, but in my case the piston was tight and I had uneven brake pad ware and rapid ware on that pistons pad. Before I replaced the calipers I could not get the air out because of the one piston that had stuck. food for thought , good luck.