Yes, a larger dia MC means a harder pedal. This is because the larger piston means your foot force is spread across a larger piston area. So a given amount of foot force pushes more fluid, but creates less hydraulic pressure. Therefore in order to get the same pressure at the slaves, you have to press harder with your foot.
Big multi-piston calipers mean you have to push lots of fluid therefore require a big MC. The big MC requires lots of foot force so you need a booster. That’s why you could stop old vehicles even tho they had manual brakes. Old vehicles had small slaves the didn’t need much fluid displacement to move. Small slaves meant small MCs ok. Small MC’s build big pressure at a given foot force so didn’t need brake boost.
It can be kinda tricky to know for sure MC diameters. If you go to RockAuto, for example, you’ll see MCs of various diameters listed for our cars. An 87 might have MCs of 3 dia’s and an 88 might have MCs of 5 dia’s, even tho all those 5 would be compatible with the 87.
It gets worse. Often you will see 2 dia’s listed for a single MC. A lot of folks interpret this as different MC dia’s for front and rear hydraulic line and think that this can be used to change brake bias. That’s not what’s going on, at least I’m pretty sure it’s not what’s going on. If you google “step bore mc” you’ll read about MC design where the MC uses 2 different dia’s in order to help the slaves quickly “take up” the gap between pad and rotor, then the smaller bore actually squeeze the pads against the rotor. This “take up” is the opposite of “knock back”.
Certainly firewall flex is an issue. I did some experiments on that a couple years ago. The write up on that is here somewhere.
Re. air isn’t the issue because my Girlings don’t have the problem. You are underestimating how diabolical a soft pedal can be. Sometimes the solution is more persistence then logic. Also, don’t be reluctant to change the MC. I know that doesn’t fit the symptoms, but after doing the same thing over and over again and not getting the results you want, at some point it becomes time to do something different even tho logic doesn’t point in that direction.
A couple years ago I convinced myself that my back slaves were not getting the same hydraulic pressure as the front. I went thru a couple weeks of being soaked up to my elbows in brake fluid as I worked with pressure gauges to test pressure every place in the system over and over again until I was sure that I had everything figured out.
After chasing all sorts of wild gooses, swapping out components over and over again, I never found the problem. Ultimately, the symptom disappeared and my braking distances returned to what they ought to have been. The whole damned adventure made so little sense that in retrospect I wonder if the “why can’t I get my damned car to stop” symptom wasn’t all my imagination.
Brakes can be diabolical.
