Disc brakes require a larger volume of fluid to move the piston than drum brakes. So a larger bore MC is needed to move the required volume of fluid. If your leg, through the brake pedal, applies 500 lbs of pressure to the larger bore piston the pressure applied to the fluid is less than the pressure that would be applied to the fluid in a smaller bore.
Therefore:
Small bore = more pressure but less volume.
Large bore = less pressure but more volume.


I guess the ideal solution would be a small bore (high pressure) MC with a long stroke (more volume).

 

Disc brakes require a larger volume of fluid to move the piston than drum brakes. So a larger bore MC is needed to move the required volume of fluid. If your leg, through the brake pedal, applies 500 lbs of pressure to the larger bore piston the pressure applied to the fluid is less than the pressure that would be applied to the fluid in a smaller bore.
Therefore:
Small bore = more pressure but less volume.
Large bore = less pressure but more volume.

Yep! But Mustang Steve and most of the trusted experts on this forum and others strongly caution against a bore of more than 1". I try to do my homework before posting, but this one is confusing for a simple carpenter.

 

Herez how I would approach this one. I would calculate the area of the piston(s) for a kelsey hayes system, then calculate the area of the pistons for your current front brakes. Since the rears are the same drop them out. Brakes work in a linear fashion e.g. pounds per square inch, you can scale your master by the same ratio as your fronts. If your front brake square inch is say 1.1 larger than the stock Kelsey Hayes, then take your master 1.1 times IN AREA than the AREA of the 1" piston e.g.( Pi X 0.5^2), back out the appropriate diameter eg Area=Pi X r^2 so the diameter = 2 X (square root of (Area/Pi))


Someone check my math and thought process - I totally just make this crap up ;o)

 

Good idea. I'll try to drop the calculations in the lap of an engineer at work this week. Sadly, I think the lack of room for proper pedal ratio and booster size is going to be the limiting factor.

 

This site has a great explanation of the math on the relationship between MC and brake calipers.

https://www.joesracing.com/master-cylinder-math/

 

Yes, I read that and similar on many other sites. The 1" bore seems to be right. I could try 15/16" but going to move the pedal pivot point first.

Once again I'm looking for the magic bullet that doesn't exist.

 

What is the pedal feel like once it "takes up" near the floor, light or moderate/high pressure required to stop ? Can you lock up the brakes currently ? If you have too much pedal travel, the last thing you want to do is go to a smaller 15/16" master. You would be undoing the effects of the pivot change. There is no reason a light car like the Mustang would ever need hydroboost unless you don't have any engine vacuum. The booster would be another area to check for problems though, or maybe there's a better booster option. (Dual diaphragm booster?)

BTW, Hydroboost only requires the 1 PS pump, they've been used on trucks for decades.

 

What is the pedal feel like once it "takes up" near the floor, light or moderate/high pressure required to stop ? Can you lock up the brakes currently ? If you have too much pedal travel, the last thing you want to do is go to a smaller 15/16" master. You would be undoing the effects of the pivot change. There is no reason a light car like the Mustang would ever need hydroboost unless you don't have any engine vacuum. The booster would be another area to check for problems though, or maybe there's a better booster option. (Dual diaphragm booster?)

Thank you for the reply. Once past the take up the pedal is firm and stops the car as well as the drums did. I'm using EBC black coated rotors and red stuff pads but I'm still braking them in at probably at 100 miles I have just gotten through the black coating on the rotors so braking is probably not optimal since I'm sure there is still black gunk on the pads from the rotors. EBC in the directions strongly warns against panic stops before 200 miles. With all that said, I don't think with the really sticky tires I could lock up the brakes at this point.

Probably 18 months ago when I researched and purchased my 7" single diaphragm booster it was all that was available that officially fit without shock tower surgery. I looked again today after work and noticed that things have changed! A company "Master Power Brakes" has put together a dual diaphragm kit that supposedly fits. https://www.mpbrakes.com/booster-master-conversion-kit/1965-ford-mustang-7-inch-dual-diaphragm-booster-automatic-trans-only.asp?catid=6DB0E5F38C1042D09A216B9C535BFDE0

Here's a link from CJP for the same thing just for grins.https://www.cjponyparts.com/power-brake-conversion-kit-dual-bowl-1964-1966/p/BBCK5/

Since I already have the adapter brackets and pedal pin mounting stuff, I was thinking about just ordering this, https://www.summitracing.com/parts/tff-2222nb and giving it a try since I'll have to tear everything apart anyway. All I need to find out is what, most likely GM, master cylinder is that. And should I increase the bore over an inch if possible?

 

Yes, a smaller bore reduces pedal effort, and in exchange for requires more travel.

 

I’m running a 1” master cylinder with my 72 disc and stock rear drums. Front pads are Autozone pads. Personally I like the feel and effort of the manual brakes. I like the modulation and feed back. Most of the time they work well except trying to stop at 100+ a few weeks ago At Watkins Glen opening day ;o

 

I’ve got sciatica on and off when my back is not doing good so the booster is a good idea for me.

 

1" is right for most mustang disk brake applications.
FWIW - too large of a bore on the M/C and you can get into a situation where you cannot apply enough pressure with your leg to lock the brakes. I had that problem with a 49 ford someone had cobbled together a disk brake system on. the M/C bore was too large and you could stand on the brake pedal and not apply the brakes fully. reduction in bore size solve the problem completely.