Thread: rear brake sizing ?

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I'm in the early stages of planning a rear-end swap for my 69 chevy 108 van....I've determined that the rear from 91-96 GM wagons (Caprice, Roadmaster, Custom Cruiser) are just one inch longer (d2d) and have limited-slip and a favorable 3.23 ratio....
I'm aware that I'll have to change the shock and spring mounts and that the bolt circle is 5on5 and I'm okay with that...
What I'm not certain about is the use of 11x2.5 brakes where 9x2.5 currently reside (both setups use 1" bore cylinders) .... As these vans don't have power brakes, and I'm going to keep the 9" drums in front, I'm not sure what to expect for brake bias ratio.....
Will the larger brakes stop better? or will they require a booster to work at all? I'm guessing that proper Front-to-rear bias will require pressure adjustable proportioning valve(s) to drop the brake pressure (front or rear?)
I'm so confused!

If the van originally had drum brakes front and rear, I suspect there is some kind of residual pressure valve plumbed to the front and the rear, to keep the shoes in close proximity of the drum inner surface. The 11" brakes will work a little better than the 9" brakes, simply because there is more shoe surface area in contact with the drum surface area, but I'd bet it will be close enough that you'd never have to jump through any hoops to make it work. I'd put the diff into the van with the 11" drums on the rear and run it. You may notice a little better stopping power and no other significant differences. It'll be close, front to rear, because the slave cylinders are the same size.

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I agree with Tech. One further thought - with 11" drums vice 9", you have a little better lever arm, i.e. mechanical advantage. Both the increased lever arm and the larger friction area should increase your rear brakes somewhat. Like Tech said, I'd run them with the swap. If the rear seems to lock up too much, you can cut a brake line to the rear and install a proportioning valve.

A couple of farm boy tricks--------if the back wants to lock up, you can put a smaller size wheel cylinder -if still and/or--you can shorten the length of the lining material on the 11 inchers so as to have nearer to the same area as the 9 inchers


also with the increased diameter, do they clear your wheels inside radius?


I'd stay away from proportioning valves and also I don't think drum brake systems used residual valves-----they came along with avent of discs-drum shoes need to be adjusted to keep clearances down

Thanks for the replies!..... I wasn't sure if those bigger brakes would work a whole lot better OR if they maybe wouldnt work as good because the geometry of the parts might require power boost to get enough fluid pressure......removing some of the friction material to the same square inches of contact is intriguing, but reducing the fluid pressure is probably a better way to go......

I wouldn't touch the friction material at all. Too much chance of bleeping it up. Proportioning valves generally will allow 57% - 100% throughput, which is plenty of range for most applications.