Is there difference between MC for power vs manual?

I am trying to determine if I got the right MC for my drum to disc Scarebird swap.
9in front drums went to disc, while keeping rear as drums. Having some issue now getting the brakes to feel "hard", instead of soft. When I researched what I needed I was told 1973 Dart would get me the correct MC for my 66 Dart slant. So I got the Dorman M80266, which had the 15/16 bore and dual well (one big , one small). I do not have power brakes, however this new MC states for Power brakes, it also states that it will only fit cars with rear disc (which I dont have).
So long question, but is there any difference in the piston action of a MC that says it is specifically for a Power system vs an MC that says it is for Manual system?
I made this list, that I thought was basically all the same MC, but perchance I am mistaken?
Dorman M80266
Raybestos MC36406
Centric 130.63017
Cardone Select 131571
Right Stuff DBMC15
BrakeBest Slect NMC11145
NAPA NMC M2122

Any thoughts?

The only thing I can tell you is that I run a smaller piston than that.

Did you bench bleed the master? Seems to me it should have a firm pedal no mater what.

gdizzle said:

So long question, but is there any difference in the piston action of a MC that says it is specifically for a Power system vs an MC that says it is for Manual system?

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It depends on the rest of the system. If the power booster operates through a bellcrank, then the leverage changes.
According to Chrysler's tech pamphlets, some similar looking master cylinders had slight difference in the front/rear piston. But aftermarket and parts store master cylinders are often consolidated to minimizing stocking. They don't worry about slight differences like the car manufacturer had to, as long as it functions close enough - however they define that.
Previous post about Master Cylinder different part numbers. How come all my manual drum cars have disc master cylinders?

I would contact Scarebird and go with what they recommend, unless there is a red flag that their recommendation is poor.

General differences:

1...BLEEDING........I'm gonna put that as a "difference" because so many seem to have trouble and it DOES have all to do with hard pedal
2...Rear drum adjustment/ to big rear drums turned out.......Try adjusting the rear drums up "hard" to lock the wheels and see if that fixes the soft pedal

3.....Push rod hole. The recess in the piston for the pusrod is different between manual and power

4....Residual pressure valves.......these are little rubber valves with springs in the ports of the master. Drum brakes have them, disk will have them in the front port for the rear drums. A disk "only" master likely will not. Google/ consult a shop manual On drums these maintain a small positive pressure and help prevent the cylinder cups from intaking AIR

5...Piston size. This depends on how much movement there is in the front pistons, how much the hoses expand, in other words "how much volume." Larger means the pedal moves less far, but you have less effective braking for a given foot pressure. On my 67, I have Linc Versailles rear disk, and Duster/ Demon front. I just "tried it." Defeated the residual valves in the stock master, and it gives me a rock hard pedal at about 1/3--1/2 pedal, and I could lock them up at 70, no problem. My legs are not very strong, anymore

So is it possible that the M80266, that states for Rear Discs, does that mean I am missing the residual valve I should have since my rear is still drums??? I thought the M80266 was on the interchange list, but ??? When I type in that part in Rockauto, it brings up all the other Part numbers that I listed in my first post. Surely they are all the same, right? Also I have installed an adj proportioning valve to the rear, and I have it wide open and it still does not lock up.

^^Don't know as there would have been few cars in that era that were 4w disk brakes. It might be "sloppy" description on the part of the seller. There is a LOT of that going around with so called 'auto parts suppliers'. I would contact them and see if you can run that detail down

If you have the cylinder in hand, it is easy to confirm. Again, google residual valves or consult the shop manual. GENTLY insert a probe like a blunt nail into the ports, no valve you should be able to insert it an inch or more with no resistance. With a valve, less, and will encounter the rubber valve. Don't force it

gdizzle said:

So is it possible that the M80266, that states for Rear Discs, does that mean I am missing the residual valve I should have since my rear is still drums??? I thought the M80266 was on the interchange list, but ??? When I type in that part in Rockauto, it brings up all the other Part numbers that I listed in my first post. Surely they are all the same, right? Also I have installed an adj proportioning valve to the rear, and I have it wide open and it still does not lock up.

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If a master cylinder states it is for rear disks, it will not have a rear residual valve. Which you do need to have if you still have rear drums- that could definitely contribute to the brakes being soft.

As far as the interchange goes, I don't know what to tell you. Sure, they should all be the same. But that depends on the person making the list, and changes to the parts by the manufacturer since the list was made. Meaning, there's no guarantee all those parts are actually identical.

gdizzle said:

So is it possible that the M80266, that states for Rear Discs, does that mean I am missing the residual valve I should have since my rear is still drums??? I thought the M80266 was on the interchange list, but ???

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Who knows. By the time stuff gets entered into the web its been copied a half dozen times, and nobody checks whether its right or even makes sense.
Now just think about this. Chrysler never offered rear disk brakes on these cars. Any M/C listed for original application for those years ('67-'74) should have a residual valve in the front outlet (rear brake feed). Earlier years had the residual valve in the distribution block, and later years used an 'expander' in the rear cylinders.
You can see the residual valve in good light. 1970 Chrysler Imperial Hydraulic Brake Service Guide from the Master Technicians Service Conference Session 274

gdizzle said:

Also I have installed an adj proportioning valve to the rear, and I have it wide open and it still does not lock up.

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Did the fronts lock up? If the answer is no, then your test wasn't close to the situation prop valve needs to be adjusted for. Follow the guidance on the instruction sheet and set it close to the factory setting. Factory setting was to initiate pressure control above 300 psi. Same booklet as linked above explains that.
Then once you get everything working correctly, if you want to further tune it, take the car out to the slickest situation you can safely test in. Adjust it so the rears don't lock up first under hard panic braking.

This may or may not be relevant, but my understanding is that many late model vehicles with rear drums (usually trucks) have the residual pressure valves built into the rear wheel cylinders instead of the master cylinder. This allows the master cylinder to be the same as ones used with 4-wheel disks.

I believe the pushrods are different lenghts for power vs manual. Manual also has an oring like retainer to keep the rod from falling out of the MC.