Just got through swapping my 1972 dart from front drum to K-H disk brakes. It has manual brakes/ I still have the drum master cylinder and it seems to work but some are saying i need an actual disc master cylinder and wondering why? (asking because i dont know). Some of the parts houses list some of the master cylinder as 10"drum OR disc brakes..like it works for both, so wondering what will be different being this seems to work? (i DID swap the distributoin block out for a disk/drum proportioning vavlve)
Thanks!

The Disc would have a larger reservoir for the front disc, it also does not have a residual pressure valve under the brass seat for the brake line as you do not need a holding pressure on disc brakes like you need on a drum set up.

The resevoiur, the residual valve and probably a small difference in the springs.

http://www.imperialclub.org/Repair/Lit/Master/232/Page04.htm

Be cause they aftermarket replacement companies are sell what is
good enough' and probably 'close enough' its unlikely anyone will take them to court when they wreck.
To convert a drum master to the disk master all they do is remove the residual valve for the disks (rear port).
You can do the same - see page 9.

Filmstrip if you prefere A-V
http://www.imperialclub.org/Repair/Lit/Films/230/

Might want t olook throughthis one as well
http://www.imperialclub.org/Repair/Lit/Master/233/page12.htm

Since you have a manual set up, get a MC with a 15/16" bore.

Like this...

https://www.manciniracing.com/ristdemacy.html

Trying to decide on if i want the newer 2 bolt MC or the older 4 bolt cast iron design.. Being its a big block dart with fenderwell headers and the headers are SO close to the MC, I probably best get the cast iron because the plastic reservoir may melt on the newer 2 bolt design..

FYI Late '70's F-bodies had power and manual disc/drum set-ups. i.e.: aluminum Cardone reman. - 101821 = manual brakes, 101822 = power brakes. I have the manual master cylinder on my race cars. My Dart Sport was a manual disc/drum car originally and it works "OK" in that application. Still sorting out my Aspen wagon. I don't know how good it would "feel" in street driving. Proportioning valve may have an influence on it all as well. I believe the piston bore is the medium size. Those M/Cs use a medium sized plastic reservoir found on full sized cars. I installed the larger version from full sized truck/van M/Cs for the extra reserve capacity. They fit but just require a tab or two to be cut off from the underside. The tabs are configured to fit their respective M/C housing, like a jigsaw puzzle.

If you go to the aluminum 2 bolt, you'll need an adapter.

Cardone manual disc/drum M/C for my Dart would be 131627.

15/16" disc master, 4 bolt cast iron

RAYBESTOS MC36406

CENTRIC 13063017

CARDONE 131571

73-75 Dart with power discs is the application.

Well, i got the front end together today and even though i have not got the correct master cylinder yet, i had to take it for a ride. I was happy with how much better it drove, BUT, the brakes barely worked! I can stand on them and the car just creeps to a slow stop. Granted, its still got the drum/drum MC on it,, but shouldnt it still stop fairly decent? I couldnt even make them lock up in my gravel driveway doing 30! It has a good solid pedal, just dont stop worth a darn..

Drum brakes of that era are self energizing.

Without going into a bunch of technical stuff, they don't need as much pedal effort for a given amount of braking force.

Disc brakes are not self energizing.

Yes there is still an issue (fluid restriction somewhere is my guess). Once I had that & it was a partially crimped brake line.

when bleeding them i can make the pedal go to the floor, pushing plenty of fluid..

Alright it does sound like there is no fluid restriction. that pretty much does point to a bore mismatch assuming the hard parts are functioning/moving as they should.

Im wondering if the pistons in the calipers are frozen up and just barely moving?

this wouldn't be the same inertia amt as a moving vehicle but I would jack it up & spin the front wheels by hand & see if the pedal stops em firmly/quickly. EDIT & you could bearhug each rim (forward) with your helper on the pedal & see if they budge (& they're up in the air if you need to go into em).

Ya, they are stuck/sticking. I pulled them off and put air to them and on one of the calipers, only one side moves and the others that do move wont hardly retract. To save some money, i thought about rebuilding them myself rather than buying calipers.. Also,I have MANUAL Kelsey hayes brakes on my 1972 dart, I see most people like the 15/16 bore MC for manual discs..well, looking at my local oreilly, all the master cylinders for manual brakes are 1.032 inch..in order to get a 1 inch bore MC i have to get one for POWER disc/drum brakes..is that ok to do or do i have to have one that is strictly for manual brakes? I would like to pick one up tomorrow..

look down in the rear MC piston "thimble" & if it has a circular recess about halfway down in there then yes it will work for manual brakes as that is where the piece of neoprene slipped on the end of the manual brake pushrod locks into. If the MC does not come with that piece of neoprene you can cut a piece of neoprene fuel line. the piece that comes with manual MC's looks the same ex for a thin groove along its length tho I dont know if that is a mfr oddity that serves no purpose or if it is there to let air excape so it will lock into the recess better & either way you can cut that thin groove your self & they oughta have a manual MC on the shelf that has one of those tailor made pieces so you can see for your self. power brake MC's do not need that piston recess but can use a MC with or without the recess. EDIT manual brakes DO need that recess