Note that we have observed that in some cases it IS actually necessary to run a residual pressure valve with rear disc brakes (!) Here is a classic post discussing this:
http://www.pro-touring.com/forum/showthread.php?t=16702 While the end result isn't listed in the post, this guy called me and thanked me for finally solving his issues for him (yes, he took the easy way out and installed a 10 lb residual valve instead of yanking the diff apart and doing it the right / hard way)...
Flying smack dab in the face of conventional wisdoms, even a 10 lb valve will not cause any noticeable drag or heat buildups, though definitely keeps the brakes "strike ready" at all times. True, you do not want to run them if not needed, or maybe only a lighter 2lb version. We typically will install a 2lb valve in each right / left hard line running to each rear caliper when a C-clip axle is in use, which cumulatively has the same effect as running one single 4lb valve in the mainline (before being split right / left at the end of the brake flex hose). Since the valves are only typically available in either 2 lb or 10 lb, running a 2lb valve x one for each wheel is a decent way to split the difference.