69 dart disk brake swap

Everything points to the "new" MC. I'd start by removing it and disassembling. Easy to do by removing the snap ring on the back. If the pistons stick, use air in the outlet ports to blow them out. First the large chamber then the smaller chamber in the front. The pistons are separate pieces. Inspect the bores and seals. You can hone the bore with one of those 3 stone tools if needed.
But....unfortunately, rebuild kits are not much less money than a complete rebuilt MC. Get a 73-76 power brake MC but run it manually. (15/16") You should also check the length of the pushrod. Mancini racing and others have adjustabies for about $30.