Anyone know off hand the piston size in a pin type early 70's brake caliper? Trying to figure the area gain going to a 4 piston set-up. Thanks in advance.

this is my opinion based on playing with brakes and not knowing your intent, but area gain will require a pressure gain. don't think that larger area by itself will give better stopping power. i'm not a hydraulics expert but picked up some ideas the hard expensive way on braking.

After a long web search, I found one rebuilt caliper listing which actually gave dimensions. 70 m.m. piston claimed. That's 2.750 diameter which is 5.965 sq./in. Looking at the Dr.Diff 4piston X 1.75" caliper kit. 4 pistons X 1.75" dia. equals 9.621 sq./in. That's 61% more piston area. The same master cylinder output pressure acting on more sq./in. is more clamping force albeit with more required volume. Just exploring options right now. Brakes are -"OK"- now, but not great.

Look up 1970 Challenger disc brake calipers on www.rockauto.com

I looked that up for sliders a few days ago and saw one brand listed piston diameters

Due to the sliding action of a single piston caliper, you must compare its volume to only ONE BANK of a multi piston caliper. In other words, all else being equal, most multi piston calipers produce less clamping force than a 2.75" bore single piston caliper.

Of course, you can always compensate with a smaller bore master cylinder.

I had thought about that as the first option anyway. Supposed to be a 1" dia. now,4 wheel drum style ,iron. Might go to a 15/16" aluminum to save some weight too. 13.7% difference.

if pressure and volume remain the same there won't be more clamping force with more area. i've been thru this. but, worse come to worse you can always swap parts back. i did a 2.75 caliper piston once with a 1" bore master cylinder; hard pedal and poor stopping. went to a 2.5" piston and a noticeable difference. tried a 15/16" master to get more pressure and it didn't work as good due to not enough volume with hard stops. settled on 1.031" master with the 2.5" piston; isn't great by any means but a lot better than the 2.75" with 1" master.

To Dr. Diffs point about the floating caliper, here's my take on it after thinking it over while eating. With the floating caliper, the fluid pressure is actually acting on 2 surfaces which apply the pads. Think of the column of fluid in the caliper as being the "center". The force pushes outward, via the piston, on the inner pad, pushing it against the rotor while at the same time it pushes on the bottom of the caliper bore, pulling the caliper body,and the outboard pad, inward toward the rotor. It's like doubling the area "doing the work". I will probably change to a smaller master cylinder first for a number of reasons. Cheaper, less work to change, weight loss, smaller bore(more p.s.i. per pound of pedal force). The car has manual brakes to which I added the factory disc front, and "C" body 11" X 3" rear drums. Got the front set-up at a swap meet and had the 11" drums from my old Newport, so not a lot of $$$ in what I have now. It's not horrible, but I think it could be a lot better.

I am not too sure what volume has to do with this unless the MC is woefully wrong for the design. Disc did require larger reservoir to account for pad wear and the larger volume of fluid required to replace as piston moved outward from disc wear.

The 4 piston versus sliding is just physics. Same as pushing on a wall. Equal and opposite reaction to force, unless you are actually moving the wall, or in this case the rotor. So you calculate 4 piston based on the 2 pistons force per side. Same for 1 piston, force on that side, is equally applied on the other side.

The 69 B use 4 2" pistons so you had 2 2" pistons on one side that actually had a larger area then the 70 KH 2.75" piston. 8 pie, vice 7.5 pie. They both used the same 1 1/8" bore MC.

The whole reason braking was better is the pad friction was improved and the pads had 50% greater friction than earlier year pads. So even though applied force decreased from the smaller area, braking force increased.

The Master training documents on the web in 69 and 70 have great info on hydraulic and design info on the brake system. Of course they were power brakes required during that period for most applications.

I'm sorry, I just don't buy the approach of using the force of only one side of a multi piston fixed caliper to calculate force. Let's say you had two porta-power rams between two fixed supports with the pistons opposing each other with a block between them. If you pump one porta-power to a given ram pressure and then pump the other porta-power to the same ram pressure, you guys are saying the second one didn't contribute any additional force?

Brain fart. I get it. In my example, the second ram is mechanically opposing the force of the first ram. If you pressurize the second ram to the same pressure as the first, you are simply substituting the mechanical opposition with hydraulic opposition, there is no additional force supplied. Got it.

keep the manual pedal and it's ratio when swapping to power brakes.
i have done that a few times, and i can tell you, it will throw you through the windshield.
manual brakes have around a 6:1 ratio, while power brakes can be 3.5 to 5:1, depending on application.[pedal ratio]

All else being equal, a 1" bore master cylinder and 2.75" bore caliper yields MORE clamping force and longer pedal travel than a 1.031" bore master cylinder and 2.5" bore pistons. This is similar to gear ratio.

Here’s a brake calculator and shows you have to calculate forces

https://brakepower.com/help_abc_27_PAC_t.htm