71 demon master cylinder

Again that looks like an aftermarket disc brake MC to me. If you want factory correct or correct-looking then that's a different ballgame. Also 67-70 & 71-forward are different. Early has a smaller 1" bore. I used one that looked like yours for an early app & it now works fine after I rebuilt it, it started leaking shortly after install, again the spotty quality thing. Front brakes go to rear of M/C. Drums require a check valve that holds a few PSI pressure whereas discs do not. For mopars this amounts to a little spring & stopper under the brass fitting in the port. You must make sure the disc portion does not have this, short of taking it apart you can check the flow with the ports open, if it has the check valve then fluid should not pour out. If your discs have this check valve installed they will drag.

Not to scare you, but my experience with replacement M/C's has been piss poor. I have a disc brake C-body & 2 of NAPA's finest quality failed not long after I put them in. I took one apart & it had a lengthwise score in the cylinder that looked to be just crappy machine work. The original made by Bendix lasted 25 years. NAPA M/C's are made in the aforementioned communist-bloc country so there's really no advantage to buying from them anymore. I gave up & sleeved/ rebuilt the Bendix to be safe. It's not a pleasant feeling to be stopped on a hill with your pedal slowly dropping to the floor & a car 2 feet from your bumper.

It is true that the parts store part number references that come up for these early M/C's are pretty screwed up, you have to double check everything.