I've been researching manual disc/drum conversion kits and have noticed that not all of the manufactures offer the same bore size M/C. Some have 1-1/8" and some offer 1". The pedal ratio of my car (65) is 6:1 when the M/C push rod is connected to the manual brake (upper) hole.
What would be the best bore size for my system when using stock 69-72 calipers?
I don't want a brake system that takes a pro football player size leg to get it to stop! I know that manuals will not feel like power brakes but I've read enough post here on TC to know that it can be done right.

What are your experiences?

GM used a 1" with those calipers with a slightly lower pedal ratio (~5.3:1) with manual brakes. Cars with that pedal and those calipers I don't think were available with manual disc.
The 67 Chevelle with manual brakes used a 1 1/8" with that pedal, and calipers with 4 pistons whose effective piston area was almost the same as the single piston ones, but the caliper was a little more efficient. The Camaros with that pedal ratio and the smaller calipers used a 1" master with manual brakes.

All that aside, I think most people prefer the 1" to the 1 1/8" with your set up, and some use even smaller, like 15/16, or maybe even 7/8.

I have used both the 1 1/8" and the 1" and didn't notice much difference. The 1" lets the pedal go down a little further but is a tad easier to press. I was thinking about changing back, but I think I'll drive it a little more first. I did notice a huge difference for the better when I changed back to organic pads.

Thank you for the complete/concise answer. Thats why this forum is the best on the net!

Can you recommend a good organic pad?

I used NAPA OEM replacements. They also have metallic and ceramic available. Metallics did not stop well on mine. I haven't checked the ceramics out.

I tried a 1" bore on my 68 Chevelle because it came with the conversion kit... Save your time and get a 1-1/8" bore. The 1" bore did not have enough volume to move the calipers with a 6:1 pedal ratio. I used a 1" bore on a 56 Chevy with a 5:1 pedal ratio and it was just right. With a 6:1 ratio the car was unsafe with the 1" bore master cyl. Just about went to the floor for even a mild stop. Panic stops were out of the question. FWIW I'm going to convert to a power booster with a 1-1/8" bore master cyl this spring as having no boost gets a little old in daily driving situations.

David, I recall reading your post sometime back. Thanks for sharing your real world experience.

I left the original manual drum master when I converted, and the stopping is so much better that it was with all manual drums, I don't think I'd ever add a booster (until I'm maybe in my 70s?)

If you go with the larger bore MC, a trick you could look into is using a single-piston caliper for a late 70's/early 80's Suburban that have a 3 3/16" piston diameter. These calipers will bolt right in place of the stock chevelle calipers, but you might need to shim the pads slightly as the throat is slightly wider for the truck rotors. The extra piston area reduces the force it takes to create the same stopping power.

I would also strongly recommend some good ceramic pads as they do make a big difference! I have used the raybestos Quietstop and like those as well as some cheaper 'store-brand' ceramics from Checker and they both stopped much better than the stock organic replacements.

Good luck,
BC

Going with larger JB7 truck calipers and same master changes the master/slave relationship in basically the same way that going to a smaller master with stock-size passenger calipers does. The difference, or trick, is that in not upsizing the rear cylinders at the same time to create a more balanced system, you are also not adding the extra travel that the rear slave cylinders would otherwise see, thus the pedal travel wont increase quite as much as when downsizing the master.

I really didn't see a huge amount of extra travel with the 1" master I used, but I did see some. I know some people here say they are happy with the 15/16" master, but I wouldn't go that far. The extra pedal travel I experienced with the 1" was somewhat alarming while bleeding, but seemed fine while driving.

But I also did not experience excessive foot pressure needed when using the 1 1/8", so you should try that one first I guess. It really is the correct size for that pedal according to looking at other GM pedal/master/slave relationships. You could compromise and find a 1 1/16" :)
GM did not use manual brakes on high horsepower applications when using the 5.3:1 pedal with the 1" master during the 70s. With the 6.2 pedal and 1.125" master, very few models were available with manual disc in 1968.

Just to throw some more fuel on...
The 1/18" does not apply quite as high (almost, but not quite) of a total ratio with the 6.2 pedal as the 1" does with the 5.3 pedal, using same calipers.
Piston area= pi(radius x radius)
(effective slave piston area)/(master piston area)=(hydraulic ratio)
(hydraulic ratio)x (pedal ratio)=(total ratio)
Most 67-85 manual brake GMs will come out to about 40-45:1 for one front brake. Fords do too. I would have to assume Mopar follows suit.

Excellent info guys!

I believe I learned something today!

Couple of questions...

First my 70 is a 4-wheel manual drum car. I have a disk setup off a 71 that I'm going to use. If I wanted to stay with a manual setup, can I keep my master cylinder? Is there anything I need to change on the MC? I have the combo valve from the 71 that I'll be using.

If I change my MC what one would I use for a 1" or 1-1/8"? Does anyone have part numbers or applications? How do I know the difference?

Next, my cousin's 67 was a manual 4-wheel drum car. His father converted it to PB, but the pedal is pretty hard, and has a long stroke. My MB car is easier to stop. I wonder if it has anything to do with the engine he has, it does have a serious lope to it and doesn't want to idle below about 900 (4spd). He's planning on going bigger on the cam because he wants 400hp out of it (327), but his father is requiring disk brakes. Guess he should go to manual disks too... Would his original manual drum MC work too, and what should he use for a combo valve? An adjustable one with the stock drum distribution block? Does it need a hold off valve?

Thanks,
David

Ack.
First off you should use a disc master if you have disc brakes. This goes for both you and your cousin. For the 71 valve on yours you'll probably have to do a little line and fitting manipulation.

The master cylinder for the 71 brakes should be 1 1/8" bore for power brakes. There are many to choose from, but your best bet is to get a booster and master which come as a unit, to make sure they are compatible. Get the master and booster correct for your year vehicle with disc/drum.

On your cousin's car, that might be difficult to diagnose on line. It sounds like maybe he has air in the system or a bad or incorrect master, judging from "long stroke", though he might also have a vacuum assist problem, or even another problem or combination of problems.
Make sure the master matches the booster, as well as he should be using a disc master. There are 2 different styles where the pushrod comes out of the booster. Long pushrod/deep access hole in master, and short pushrod/dimple master. If the short pushrod is mixed with a deep master, you will have a long stroke and insufficient braking. Most aftermarket boosters will have the short pushrod, while many stock masters, esp drum, will need the long pushrod.