13" Mustang brakes, pedal feel?

Hard telling with engine vacuum and booster, maybe the rebuilt booster is not performing as expected?

Is it because you have not broken in the brake pads?

Or is the stock brake proportioning valve or distribution block causing a pressure issue with fronts?

It's possible that the calipers are using about 2x as much fluid. 4 pistons (Ford) or 6 pistons against 2 (AMC).

I assume I will be using similar calipers, just never got around to doing so, as there will be more options to choose from by the time I get my hydroboost setup.

For myself, if I were to use the caliper setup, It would use a master cylinder that is more compatible. I think with any AMC brake system, dropping in newer calipers will effect how braking is achieved.

I've been running a Ford Ranger master cylinder and booster with my four wheel disc setup for about 14 years now. It has a hard brake pedal too... feels good when first applied, but gets hard real quick as you press. You can change the pedal ratio or master cylinder bore to change braking effort. The first can be hard -- you have to change the point where the pushrod connects to the arm. Might not have room for much change. A smaller bore will increase pressure to the brakes but increase pedal effort, a larger bore will decrease effort but also decrease pressure. You could probably go to a bit larger master cylinder with the booster and not have an issue with pressure. AMC typically used a 1" bore master, though some of the brake systems (mainly the 75-78 big Bendix calipers) used a larger MC bore -- 1-1/8" IIRC. A 75-76 Hornet MC would do the trick (77-78 big Bendix only used in the Matador). If the hard pedal is an issue I'd try that MC first.

Red Devil wrote: What is the effective piston area of the Mustang caliper vs. the stock caliper? A multi-piston caliper may have less effective area than a stock single-piston caliper, so needs a smaller bore master to match stock clamping force, pedal effort and pedal travel. When calculating effective area, use the total piston area of one side of the caliper, i.e. single piston floating = area of single piston; 4 piston fixed = area of 2 pistons (opposite side provides force balance) Keeping same pedal ratio: *smaller bore master = increased line pressure = increased caliper clamping force = increased braking torque (... or reduced effort for same braking torque) *smaller bore master = increased pedal travel (more travel needed for same volume) *larger piston area caliper = increased clamping force = increased braking torque = increased pedal travel at same pedal push force and same master cylinder *higher CoF pad = increased braking torque at same pedal push force *larger diameter rotor = increased braking torque at same pedal push force Keeping same caliper, biggest changes will be master cylinder and pad CoF. Hope this helps,RD

Thanks every body for the info. I didn't get much time to research and do the math but it looks like the stock calipers were single piston@2.596", and the Mustang calipers are 2 piston@1.68"

While tomj's math is correct, there are a couple more factors to consider:

6t8amxetc is using 13" rotors. I'm assuming these rotors have a larger radius where the pad clamps on the rotor compared to the original braking system. For a larger radius, the braking effort would be increased for the same pedal pressure. Or if going for the same braking force, less pressure (and pedal effort) would be required. this would lessen the required step down in MC bore size tomj's math shows above.

We do not know what the tire diameter is relative to what it was (if new wheels and tires were mounted). Larger wheels will require more pedal effort for the same braking effort, smaller will require less. Basically what is the ratio of tire diameter to braking diameter relative to the previous set up. These two directly play off each other.

Although it is small, even rotational inertia of the wheel plays a part, albeit a smaller one. Angular velocity and effective mass radius are the two driving factors for this component; you are not only stopping the mass of the car, but the rotational inertia of the wheel/hub assembly as well.

And are the pad friction coefficients different? That also changes things.

I've kind of reiterated what Red Devil posted about and added a few things. By no means am I saying tomj is incorrect, just mentioning that there is more to that particular calculation than what was presented. Heck, there's even more math than what I've laid out! But everything mentioned by Red Devil, tomj, farna, and myself covers most of the simpler components of the calculation.

HOWEVER, based on your original statement of pedal effort being higher than you would like, yes, stepping down your MC bore size as shown by tomj will provide the same braking effort with less pedal effort; but the trade off will be a longer pedal stroke. Or changing the pedal ratio as farna said.

Basically, there's a great deal of math that goes into a braking system design. But the math is all centered around pedal effort vs braking effort. Braking effort is easy to define - have to get the car stopped at a reasonable pace. But pedal effort is completely subjective and the desired effort depends solely on the driver's preference. So if you have to press too hard for your liking, then the two easiest changes would be decreasing MC bore size or getting some pads with a higher CoF. A slightly more involved (in my mind) change would be the pedal ratio. Outside of that, it'd require replacing the parts you just put on your car. And I doubt you want to do that.

Hope this helps.



Edited by RTTComanche17 - Jul/20/2017 at 10:31am

Thanks guys! Lots of good information here. Now my only problem will be finding the correct master cylinder that will fit. It is very tight running power brakes on this Hornet body. Plus the brake line ports need to be facing the engine. It is all pretty tight. As far as the Pads go. I am running the Hawk HPS pads. They have pretty good grab. So I believe the master cylinder is my best choice at this point.
Thanks again...

Edited by 6t8amxetc - Jul/20/2017 at 10:00am