Proportioning Valves?

A combination valve is a proportionaing valve that incorperates a residual valve to maintain apx. 2psi for the rear brakes. the proportioning valve just limits the front to rear ratio.
When i did the disc swap on my '56 I went through Master Power and was realy pleased with the products they have....I'm still kicking myself in the *** for being cheap and not getting the combo valve mounting braket (worth the extra). I didn't feel like screwing around with the manual adjusting knob. On the down side in your case if you get the combo valve for a disc/drum setup you'll need a new one for an all disc set up later.