Hy Mearl, because the master cylinder you're using was designed for a disc/disc application, both chambers would be close to the same volume, if not identical. Wherever possible I try to copy factory engineering, the only exception to that would be to make something bigger/stronger, so I would use the same size lines as the factory used. I still think you should bleed the master cylinder alone, before you attempt anything else. Once we went to disc/drum vehicles, you are correct the larger volume cylinder was used for the front brakes, on the drum/drum master cylinders the primary system activates the front brakes. I think you may have some air trapped in the master cylinder. I still recommend the installation of a combination valve, but that is ultimately your choice, good luck, and let us know how it goes.
