Hy jss1956, if the master cylinder was for a disc/disc system the reservoirs should be the same size so capacity should not matter, however most vehicles with disc/drum brakes have the front [disc] brakes plumbed into the primary side of a dual master cylinder. Does your combination valve still have the ability to shuttle over and seal off a leaking system, or has it had the valve locked into position? If it is not locked in position, it may have shuttled over to seal off the front brake system during the bleeding process [the system with no pressure while bleeding]. There were some tools made to lock the valve in position while bleeding. If your valve still has its electrical connection and it is a one terminal setup, check for continuity between the terminal and the body of the valve, if it is a two terminal type, check for continuity between the two terminals. If you get continuity on either of those checks, then the valve has shuttled over, and must be recentered before you can get much if any fluid through the sealed off system. To do this you must open the rear system while generating a pressure in the front system, you may have to plug the port right at the combination valve to accomplish this. Gently bring up the pressure and check for continuity, when the continuity goes away the valve is in the center. I would try and purchase the locking tool for your valve and keep it in place until all brakes are bled. I would also suggest that you plumb the front brakes into the primary reservoir, hope that helps.
