I think Mike is correct, it may be worry about nothing.
It looks to me like the risidual valves are really aimed at preventing excess travel in the brake pedal.
Wilwood provide write ups about their 2lb and 10lb valves for use in drum and disc applications which seem to provide a pretty clear explanation of the use.
Have a look here
http://www.wilwood.com/MasterCylinde...derValves.aspx just scroll down the page a bit.
I guess if doubt remains an email to Wilwood disguised as a pre purchase enquiry might shed some light.
I am using the Sierra MCyl designed for drum rears (which is what i have) I know a couple of the guys in the MOC were using the same cyl/brake configuration and then replaced the drums with Sierra rear disc setups. I haven't heard of any issues after they upgraded. The only comment from the MOC BMW Cabrio tech rep was that his car was borderline overbraked at the rear when tested during SVA but I don't think he has had any issues as such.
John