I had 3/4 rear cylinders on Barney with S front brakes and no booster. After a major drama on a freeway in the wet at 110kmh (rear wheels locked then car weaved during heavy braking)) I fitted 5/8" rears.
That's the reason the Cooper S had 5/8" rear cylinders, SAFETY. Not to get a firm pedal.

Not sure I understand what is going on here. The cooper S slave is a 5/8 bore and the standard one is a 3/4 bore which is equivalent to a 6/8 bore. The Master Cyl emits a said volume of fluid. In a larger bore (the 3/4 one) it requires more fluid to move the pistons of the slave outwards due to the increased volume of the slave cylinder. Therefore you need to depress the pedal further in order to actuate the slave to the same distance as that of the smaller diameter 5/8 slaves. If this is the case with fluid dynamics then yes switching to a smaller diameter slave will require less pedal travel to activate the rear shoes. You would be more likely to lock a rear wheel with a small cylinder than a larger one. You can apply more pressure and better braking affect on the rear wheels with the 5/8 cylinder.

There is no doubt that changing to the 5/8 cylinder WILL reduce pedal travel.

Why you guys are locking rear wheels with 3/4 slaves is anybodies guess, but I would say it is more likely because of a lack of maintenance and your rear drums are out of round. Somebody please correct me if I am wrong.
Greg

Bigger cylinder has more surface area, force at shoes = pressure x area. So brakes lock with less pedal pressure.

I don't think you need to go that drastic to get firmer pedal feel. Back brakes cylinder size don't have much of an effect to higher pedal, lots of other stuff does. Suggest you look at those first. I recently overhauled 90% of the brakes on my car, every little bit counts. This is what I did and I have excellent pedal feel:

- New master cylinder (You can rekit yours, but my tin tank broke it's lid screw part)
- Replace the rear adjuster screws and wedges <--- this made the biggest single difference other than the MC change
- Replaced front and rear brake hoses <--- this is important, they do internally bulge apparently and give you bad pedal feel
- Replace front caliper seals, better still just rebuild the front calipers
- Replace clevis pin and generally get rid of all play in the pedal
- Change to Green Stuff pads, I had black stuffs and the Greens are superior bite and fade much less
- Bleed the system properly... There are many ways to do this and the single best way I find is still to have someone step on the pedal method
- Last but not least, check and replace the rear slaves, check the drum wear and shoe wear, adjust them properly with the new wedges and adjuster screws

If everything is in tip top shape, you should have excellent brakes. Going to bigger rear wheel cylinders is sort of trying to lipstick on a pig as the old saying goes.

If after all this you are still dissatisfied with pedal feel then it's time to get a booster!

Kevin,
Are you kidding. Would you like me to do the maths for you?
Sorry that was harsh, p1v1/t1 equals p2v2/t2 given temp is constant the pressure is inversely proportional to the vol.

Sorry Greg but Doc's right (Pascal's Law for fluids). You've used Boyle's Law for gases...