I have a 1965 Coronet that I'm putting 2.5"x11" rear drums on. It's a manual brake car and this hasn't come up before. What size bore for the wheel cylinder should I use. They offer a 15/16" and a 1/125" bore for the wheel cylinder. I know with the master I'd use a 15/16" bore but now I'm confused a bit on which bore to get for the rear wheel cylinders. I'm going with discs up front.

Thanks

The larger bore heel cylinder will apply more pressure to to the shoes but will require more fluid than the smaller bore.

The inverse applies to masters IE: a smaller bore master will deliver more pressure with the same force applied, but deliver less fluid requiring a bit more pedal travel

Thanks, I've just never been given an option on wheel cylinder bore sizes and wasn't sure if it was supposed to be a power vs manual brake thing.

Smaller is better on drum brake wheel cylinders
Been there done that, put bigger wheel cylinders 1 1/8 on the back and that car didn't stop worth hoot after that. Put the 15/16 back on it started stopping way better, did not change master cylinders either

Thanks Cab

There is also a 7/8” bore wheel cylinder that works with the 11” brakes.
It can be used to reduce the braking power a bit to balance things out if the 11” drums lock up too early.
Kind of similar to using an adjustable prop valve to balance the brakes out.Raybestos part number wc37236

I installed the 7/8 bore wheel cylnder with 11" drums on an 8 3/4 rear end that I built that went into a Scamp. The owner was happy with how it well worked.

From a hydraulic engineering viewpoint that doesn't make sense as the larger bore wheel cylinder will apply more pressure on the linings

Thank about that, how can a bigger volume wheel cylinder put more pressure against the brake shoes if the master cylinder size remains the same when you increase the wheel cylinder size
I've swap master cylinders sizes from bigger to smaller and got better stopping with no other changes doing that with discs in the front and drums in the back

I got 15/16" bore AC Delco, picture showed Made in USA.

Box says Made in China.

Thanks

Because there's other factors involved. If the pedal stroke was held constant, you'd be correct. If stroke isn't held constant then the larger cylinder will apply more force on the shoes.

i use a 1.032 bore master cylinder and 7/8" rear cylinders in my '65 coronet manual brakes. i couldn't tell any difference between to stock rear cylinder and the 7/8"; but master cylinder is a different story. don't use the 15/16". i done that before and it didn't work well; too much pedal travel. i would only use a 1" bore or the 1.032" bore for the master cylinder.

in the world of Fluid the following applies, lets just put some numbers out:
The formula for pressure is force divided by the surface area.

If the master cylinder has a 1-inch bore, the piston’s surface area is .78 square inches. If you apply 420 lbs. (70 lbs x 6:1 pedal You divide the output force of 420 lbs. by the surface area of the piston, you would get 538 psi(420 lbs. divided by .78 inches) at the ports of the master cylinder.

If you reduce the surface area of the piston you, will get more pressure.

This is because the surface area is smaller, but the output force from the pedal stays the same. So, If you used a master cylinder with a bore of .75 inches that has a piston that has .44 inches of piston surface area, you would get 955 psi at the ports for the master cylinder (420 lbs. divided by .44 inches).

the pressure produced by the master is multiplied by the surface area of the piston at the shoes or caliper. So a smaller piston will aplly less pressure while a larger one will apply more.
One possibility is that the added pressure from the larger bore W/C was causing the backing plate or drum to flex resulting in a loss of surface area contact.
Research what i have said via google and you'll find I'm not blowing smoke

Try doing both on one of your cars and report back, please

I have.

On my trucks I upsize the wheel cylinders for more rear braking. There's a small increase in pedal travel but it's clear that the rear brakes are doing more work.

Popular upgrade on older D3500 dually is to use the larger GM rear wheel cylinders. Size change was from 1 1/16" to 1 3/16". It does stop better with slightly more pedal travel. Nothing else was changed.
Doug

No need to my brakes work fine. but I'll ask yo to read up on hydraulic brakes and how the piston sizes affect things. Both the master and cylunder/ caliper.
They are inverse of each other. here's a site that explains what I am saying
BRAKE SYSTEM PRESSURES vs PISTON SIZING

I used the 7/8" as well, this advise was given to me by Dr. Diff.