When i put a vts rear beam on my ax I had real problems with the oem bias valves, imo theyre pretty naff as standard. When you lower the beam the bias valves sees that as extra weight in the rear so brakes harder - when the rear locks up before the front youve got problems.
Aftermarket bias valves are good but have downfalls, you need to ditch the split system, and pipe the front pair together and the rear together rather than diagonally a rear and a front together. It means faffing with the brake lines, also if you do have a break in the lines then your gonna be left with just the rears or the fronts... and thats an MOT fail