You should have one or the other, the reducing valves in the engine bay, or the laden valves on the rear beam, not both. These two just work on different basis. The laden valves reduce pressure when the rear wishbones move downward. The XR3i/S1 type valves on the other hand react on decceleration and they can be tuned by varying their angle. On top of all this, the rear drum cylinders are available in various different bores to find the ideal low and high grip brake balance.