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.