Disc Brake Conversion Failing

Yeah I seriously doubt the reservoir size is what's causing the issue here. Unless the intake holes are sucking air because the fluid level is too low with a full brake application the reservoir size is not super critical. The line pressure is determined by the bore diameter of the master cylinder, and ANY of the mopar MC's should effectively activate the brakes because it's only a swing from 15/16" to 1-1/32". Pedal feel will change in that range, but all will stop the car. And the residual pressure valves shouldn't be an issue. More importantly, it's really unlikely to be the issue unless the MC is actually defective (which it could be…). If it didn't have a rear residual valve it would be difficult to get either of the rear drums to lock up. If it had a drum residual on the disk side then the front disks would drag, which isn't the current issue.