I have used 10# inline RPVs on low drag fronts with no problems, it cuts the piston retraction back to about .010" from .020", so you don't have a brake drag problem with that setup. I don't know what RPV rating is in the MC you have, stockers can be as much as 16#, that extra residual line pressure is what might be making the rear brakes drag in your case, with your current setup.
You can test the RPV, the setup is simple.
Adapt a 0-30 psi gauge to the outlet. (in this case I am testing an inline wilwood)
To test an RPV in a mastercylinder, just adapt a gauge anywhere downstream from the outlet, youcan even plumb it into a bleeder on the caliper.
Apply about 20 psi, the let off the pressure...The gauge will tell you what the residual pressure rating is.
That is a valve that I took apart and installed a better spring, it now holds about 12 psi. I test them for pressure, as well as time it takes to leak down. It should hold indefinatly, any less than 20 minutes at the rated pressure and I would consider the valve junk.
Yes, as you can see all of my info is gotten from books.