Took the 70 Duster with 10" drum brakes out of winter hibernation today. It started right up as usual, ran perfectly but at the first stop light I could feel the dreaded slow sink to the floor of the brake pedal. So it's new or rebuilt master cylinder time. The attached picture is the one that's on it now, but I'm fairly certain it's supposed to have the one with the bolt and the metal piece that pushes down each side of the lid. So, where to get one.....Rock auto shows the one with the bolt down lid without the metal piece that pushes down on each side of the lid. My experience with these is that the lids never seal right and leak. On the one like I have on it now the lid seals perfectly but I'm not sure if that's the one that's supposed to be on it. Any advice on the type and where to get the right one with good quality and a lid that seals will be greatly appreciated.

One thing I should add is that the car was originally a slant 6 car with 9" drum brakes and when I converted it to the 360, I used the V-8 K member and entire front suspension including the 10" front brakes from a donor 71 Dart with a 318. I also used the entire 8 3/4 rear including the 10" drum rear brakes from the donor 71 Dart. So maybe I should be using the master cylinder for a 71 Dart with 10" brakes and this looks different from what's specified for a 70 Duster with 10" brakes.......thus my confusion.

BTW, I care much more about functioning properly than I do about original look.

I never had much problem with those lids leaking, but, I left the fluid level slightly down rather then having it right up to the brim at the lowest point, and then I made sure the top of the master was dry, and the bottom surface of the rubber was dry. Those two things solved most of those leaking lid stories. If there was still a leak, I would check to be sure the top of the rubber and the bottom of the lid was also dry. If there is brake fluid between the metal and the rubber, you have created a void for more fluid to leak out, especially if the revivor is filled to the brim.

That’s disc brake master cyl. Mine has the smaller one with equal bowls and bolt to hold lid on. A long time ago the other 70 I made into a race car had the /6 with disc brakes and the smaller master, original because it still had the paint dabs on bolts, this was 1968. My 69 M-Code had 10” drums and smaller master

It seems most parts houses show the bolt on lid master cylinder for the 70 Duster with 10" drums and the master cylinder like is shown in my picture above for 71 Duster with 10" drums. Why the difference?

The bigger chamber-bowl is made for the front disc brakes (because it needs more volume) if plunger is same diameter it makes no difference so they stock just one size. My 72 Duster has power disc with master same as yours. The race Duster was the best stopping car I’ve ever driven, front disc and small cylinder no power. If your’s is working now I wouldn’t change it.
70 and 71 are identical

The master design change from a bolt on lid to one with a bail , but it has 2 equal size pots. Also I'm pretty sure drums masters have a built in residual valve that the report of the disc drum master does not.

Drums look like this https://www.rockauto.com/en/moreinfo.php?pk=309083&cc=1231939&pt=1836&jsn=875 ... this is listed for a 1971 Duster doesn't say 10" or 9" but it does have a bore size that is bigger than a 9" listed master.

I just looked at rock auto and they have a bunch of disc masters listed as for disc OR drum ... after my experience with them having the wrong shocks listed for my 98 Ram 2500 and them thinking that they can't possibly have a wrong listing so I had to pay return shipping for their F'up... I don't have a lot of faith in their listings.

Thanks. That's the exact one I ordered last night.....the one with the wire lid hold down but two equal size ports. It was the one listed for a '71 Dart with 10" drums which is what the donor car was that gave up it's brakes and K member to my Duster almost 25 years ago.

Thanks for the help.

sorry i didn't reply sooner, but i had tooth surgery [yanking out if you prefer ] and i have been kinda flat since. i'm slightly better now.
anyway, for reference, the wagner part number from my master catalog for your duster is : 506207
that fits from 1967-1970 with 10" drums, but the catalog doesn't list the bore size. however, i "think" it's 1". this is based on the 65-66 master used with 10" drums, manual or power, showing a 1" master being used.
remember, those years are a SINGLE POT master, so i could be [more than likely] wrong.
sure wouldn't be the first time, and won't be the last either !

Finally had some spare time to get the master cylinder swapped out...........old one in the first pic and new one installed in the second pic.

[quote=cudaman1969]That’s disc brake master cyl. /quote]

Correct. The one removed was a disc brake master cylinder.

Agreed,
And just for reference.

The one installed is 1971 and later

1967 thru 1970 used the bolt down lid .