The check valve (actually a ~10psi residual pressure valve) is designed for drum brake systems. If you're going to run disc/disc, you don't want an RPV in either leg of the system.
BUT -- depending on what master cylinder you're starting with, there may not be an RPV in the from (or rear) outlets. Stock disc/drum master should have an RPV in the rear outlet only. Drum/drum master would have them in both ports. BUT, a lot of the master cyl rebuilders don't bother re-installing the RPV's, so a rebuilt "disc/drum" or "drum/drum" master may not have any RPV's at all.
Find an apropriately sized drill bit and use the back (blunt) end of the bit to probe the outlets -- you should be able to feel the RPV's if they're in there. Remove any RPV's from the m/c if you'r going over to disc/disc.
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