Disc Brake Conversion Failing

Had similar issues, the front brake line at the master has a residual pressure relief valve, make sure it has been removed for your front disc brakes, the valve is for disc brakes, even though I had replaced my master with disc front rear drum the remanufactured mac still had a residual valve, I cant recall who on here brought it to my attention, but I removed front line, screwed a sheet rock screw in and pulled the residual valve spring out, tah dah, fixed, when I tell you many dollars in new masters/ calipers/portioning valve and brake lines to get to a simple fix. Check it out, you may be pleasantly suprused!
OH!!! I really missed this - if the OP's replacement master really is a drum/drum MC, then it would come with residual pressure relief valves in both the front and back outlets which could be a great source of problems. So it's not just the reservoir size that is the issue - the unequal sized reservoirs are an indication that the MC is for a Disc/drum system with the residual valve only on the back brakes...