66 valiant disk brake conversion??

65 Mopar said
I went to the auto parts store yesterday and ordered hoses, pads, and a master cyl. I basically ordered the same parts another member had listed... but according to his post, the mc has a 15/16 bore. What difference would a 15/16 bore make compared to a 7/8 bore? Should I have my parts guy get me the 7/8 instead?

The pedal effort for 15/16 will be slightly greater than a 7/8. But it is not worth worrying sbout. There also may be a slightly different proportioning ratio between the front and rear piston diameters.

I have a 68 cuda with manual front 73-75 a body disc's and 10 X 2-1/2 rear drums. I've tried 7/8, 1-1/32 and 1-1/8 on this car with this brake setup. Overall, the feel with the 7/8 was the best. The 1-1/8 gave a VERY hard pedal, too stiff for my taste. Currently using an old Direct Connection 1-1/32 aluminum MC mostly to save weight and it's fine. You can easily measure the bore at the rear of the MC where it meets the firewall.
You also need some means of retaining the pushrod in the MC piston. Either the rubber o-ring or the bolt-on style will do.

I believe simple is better and eliminated the factory safety/proportioning valve. If basically does nothing but turn on the red dash light if there is a leak. My foot can tell me that as the pedal will drop if there is a leak. I plumbed the large chamber of the MC into a simple brass tee on the frame rail. One outlet to the left front and the other to the right front. The smaller MC chamber goes into a Wilwood Summit) adjustable cylinder on the frame rail under the firewall. The outlet goes back to the line feeding the rears. While I was at it, I plumbed a Line-loc into the fronts on the cuda, but not on my /6 Valiant.

It's simple and already set up for practically any future brake system I throw on either of these cars. For instance, I got the Val running last fall with the 9" drums but acquired front K-H disc's over the winter. No change in plumbing required except swapping to a disc brake MC. My 64 Val had a single circuit MC but I used a dual circuit MC for a 67 drum brake Cuda when I got it running last year.

I believe your "brake hold off valve" is not required as it's built into a disc brake MC.