Now it time to talk about a little technical problem I created for myself.
Warning: This is very long post!!!(for me at least)
Anytime you make changes to a build after you're well along in the construction, you're subject to create problems and that's just what happened here. I do want to add that I'm not an engineer and this is just a long story about what I did to my car. I'm not going to tell you this is what you should do (only you can decide that).
I started this build with GM metric disc brakes on the front and S-10 drum brakes on the rear. Since these components were designed to work together by GM, they would have been a pretty good setup. The problem cropped up when I swapped the repro 39 Lincoln drum brakes on the front.
Now I had huge relatively low pressure 12" brakes designed to stop 2+ tons of Lincoln (with no booster) on the front and 9 1/2" brakes on the rear designed to work in a high pressure boosted disc brake system. The front wheel cylinders were 1 1/8" bore, the rears were 3/4" bore and the master cylinder was 1" bore. Throw in a pretty good difference in rubber size and it was obvious that I had a bad mismatch. I figured that with the boosted master cylinder and self-energizing drums front and rear, the brakes were going to be overly sensitive and the fronts were going to be locked up before the rears ever got enough line pressure to do anything meaningful.
My first thought was to swap to bigger drums on the rear. That would have easily solved the brake mismatch issue, but would have added a lot of unsprung weight to an already heavy rearend. T's don't have a good sprung/unsprung weight ratio anyhow and I didn't want to make it worse by adding more weight. Besides, the S-10 drums and other parts I had were in perfect shape. All I really needed were new wheel cylinders. So, I looked at other options.
I decided to do a little research on drum brake systems from back when they were pretty much the only game in town to see how they were setup from the factory. The NAPA website lists the bore sizes of brake parts, so I spent some time checking out brake specs for several cars and trucks from the 50's and 60's. I did notice a pattern (sort of) in the sizes of parts. I say "sort of" because there are always exceptions. The front wheel cylinders and master cylinders were generally the same or nearly the same bore. Manual brakes had slightly larger wheel cylinders on the front, while power brakes got pretty much the same size as the master. The rear cylinders were about 20% smaller (by area) on trucks and 10-15% smaller on cars.
Using my research as a guide, I looked for a better combination of parts. Since my master cylinder is 1" bore and has a booster, I looked for some 1" bore wheel cylinders for the front. A late 60's Buick 225 has 12" brakes (turns out that didn't really matter) and 1" wheel cylinders on the rear that matched the castings and port locations of the Lincoln brakes I was using. They were a direct bolt-in replacement and brought me in line with my research.
The rear wasn't quite as easy, though. The rear cylinders are of a much later design than the fronts and had a unique setup. The pistons and the extensions that push on the shoes are one piece. The only wheel cylinders I could find that were larger and had those pistons were from a 90's model Astro van. They were 7/8" bore. That's still almost 30% smaller than the fronts. It was time to go to plan B.
I found that an early 80's Caprice had 15/16" bores, the same port layout and bolt spacing, but of course the pistons were wrong and the machined locating boss was 1/8" too big. Hey, that's no problem since I have a lathe and I'm not afraid to use it!
I fabbed up some new steel pistons with the correct shape and size. I also turned the wheel cylinder boss down and pushed the shoulder back to get the center line of the bore back where it needed to be.
These new cylinders are 19% smaller than the fronts and that's a lot closer to my research. Not as good as I had hoped, but in the ballpark. The next size up would be 1", but I don't want to worry about locking up the rear wheels first on slick roads (or dry roads for that matter).
I could have left the 1 1/8" cylinders in the front and went with 1" cylinders in the rear (modified 85 Chevy truck), but I would have had to change the master cylinder to a 1 1/8" unit. That would have given me a 14% difference, but the master cylinder costs more than the wheel cylinders.
I'm going to leave it as it is for now and see how it works. I can always make changes later. Since the new rear cylinders have an area that is 36% larger than the old units, there's no doubt that these mods will improve the effectiveness of the rear brakes. The smaller front cylinders should make the front brakes less sensitive. The only problem will be if I created a premature lockup problem in the rear (that won't be good!). I'm not overly concerned about that because the difference in drum and tire sizes still has the rear brakes at a disadvantage. Time and testing will tell, but I'm moving on to other parts of this project for now.
Top cylinder is the stock Caprice unit, bottom is the stock S-10 and the middle one is my new hybrid.
Just a little action shot of the lathe work.
