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Brake Pedal Travel, Firmness and Hydraulic Sizing

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captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
Pulling from another thread. One of the things I'm struggling with on the racecar is getting the brake pedal throw just right to enable quick heel toe and allowing me to focus on braking deep into zones and smoothly transitioning into turn-in.

Right now, the current iteration of braking system of the car is:
S550 OPmustang 15" race kit with Performance Pack calipers, Girodisc 380x32mm discs, fresh G-loc R18 pads
Kohr Race booster/master setup (Which is some form of early S197/Pre 2011 booster/master)
(see the other thread for discussion of taper on pads)

Given the pedal travel is so long and a bit soft, its difficult to hit the heel-toe on the car. Last weekend I basically added 3/8" spacers to the brake pedal cover to lift it up which shaved 2/3 a second off my laptimes, but still wasn't ideal. I could have gone 5/8 even, but I'm still having issues grabbing either enough brake without grabbing enough throttle to defeat the brake, or being too far over and not being able to grab the gas (but getting an excellent grab on the brake).

The problem being that the depth needed for the first portion of braking requires non-human amounts of foot rotation to grab the gas if you position properly on brake with your foot. I'm very good at heel-toe, especially on the S550 (where I even spaced up the gas pedal).

I think ultimately I have too small a master cylinder diameter, and not sure what I can find for S197 that would fit. Research of other masters show that they all are basically the same (and the calipers were built around the fixed master size then boosters were different to get the target feel).

I'm considering seeing if the S550 performance pack master and or booster would work, as even though its a bit on/off in modulation, it is nicely sized and firm for the car. The firewall is different between cars, but the parts look very similar (ports on opposite side of master but not end of the world to replumb). Another thought is to get another early S197 master, and find a machine shop to bore it out to a new cup size and put new cups and seals in it. I like that from a fabrication perspective and making it work exactly for my needs, but that's just one more potential point of failure on a critical system on the car.

See below for other comments from my other thread.

Long pedal/overboosted sounds like you have too much piston volume in your new calipers. That could also mean your brake bias has changed for the worse.

Bias aside, You either need a larger master cylinder to better match the increased volume of your calipers, or pick a caliper that has a properly sized pistons for your brake system.

Is it a new caliper? If not you may have a sticking piston. 2pc rotors help a little.

unfortunately there's no master cylinders I can change (05-09 all had the same size piston internally from my understanding). I'd love to investigate retrofitting the the proper S550 master/booster but I imagine that takes a lot of fabrication.

That said, calipers have about a year on them, they were new.

The respective brembo caliper piston specifications are:

Piston Diameters (mm)
05-14 GT: 44, 40
07-12 GT500: 44, 40
13-14 GT500: 38, 34, 30
2015 GT PP: 36, 36, 36

Piston Diameters (inches)
05-14 GT: 1.73, 1.57
07-12 GT500: 1.73, 1.57
13-14 GT500: 1.50, 1.34, 1.18
2015 GT PP: 1.34, 1.34, 1.34

Piston Area (one side of caliper)
05-14 GT: 4.30 in^2
07-12 GT500: 4.30 in^2
13-14 GT500: 4.26 in^2
2015 GT PP: 4.73 in^2

The increased piston area of an S550 caliper will result in longer pedal travel, but the system will generate more clamping force if you do not run out of pedal travel (i.e. pedal contacts floor).

But, you also changed your master cylinder. If the new master cylinder has a smaller piston than the one it replaced, this will also result in a longer pedal.

Find out what the piston size of your current master cylinder and the size of the original unit. I suspect you might have installed a smaller master cylinder while at the same time moving to a front caliper with greater piston area and volume.
 
"Find out what the piston size of your current master cylinder and the size of the original unit. I suspect you might have installed a smaller master cylinder while at the same time moving to a front caliper with greater piston area and volume."

So if I'm reading this correctly, a S550 swap should require a 10% larger master cylinder area. Did I say that right??
 

captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
S197 (all models): 1.063"
S550 380MM system: 1.125"

So 6% larger bore (whereas the total piston diameter is 9% larger), but that's a lot closer. I'm assuming small changes make a big difference here.


Unfortunately, the master cylinder mounting flanges are on opposite corners than they are on S197 Mustangs, but the booster looks very similar. If the pedal side dimensions are basically the same or just even close enough to get the pin on then might be in business. I'd also have to measure and obtain fittings. Maybe even make new hardlines to the ABS module or see if I could make adapter lines to the existing ones
 
492
387
DFW, TX
The area of the bore is .887 in^2 vs .993 in^2 so that's 11% increase master cylinder area for a 10% increase in piston area in the brakes. Sounds like a winner to me.

**************************
Note:

The S550 PP piston area previously shown is correct, but the piston size in inches is not.

if: 2015 GT PP: 36, 36, 36 mm
then it does not equal:
2015 GT PP: 1.34, 1.34, 1.34 <---- this should be 1.417"

36mm/25.4mm per inch = 1.417"

or 3x1.577in^2 but it reached the correct final area above of 4.73 in^2 which is the area in chart above
**************************
 

captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
The area of the bore is .887 in^2 vs .993 in^2 so that's 11% increase master cylinder area for a 10% increase in piston area in the brakes. Sounds like a winner to me.

**************************
Note:

The S550 PP piston area previously shown is correct, but the piston size in inches is not. That was throwing me off. The final area number is correct though.

if: 2015 GT PP: 36, 36, 36 mm
then it does not equal:
2015 GT PP: 1.34, 1.34, 1.34 <---- this should be 1.417"

36mm/25.4mm per inch = 1.417"

or 3x1.577in^2 but it reached the correct final area above of 4.73 in^2 which is the area in chart above
**************************

Oh man I’m super dumb today. Good call on comparing the surface area of the piston.

Thanks!
 
898
544
Yes, diameter is supposed to be 1.417. The original referee I looked up incorrectly listed the size as 34mm / 1.34”. But, the pistons are actually 36mm. I used 1.417” to calculate the piston area.
 
898
544
I think you might have a 2008 master cylinder based on what I saw on Steve Esposito’s car. If so, that master uses a booster with a longer pushrod. I wonder if the pushrod length and pedal ratio is part of the problem?

Anyone know if the 2008s had a different pedal assembly or booster pin location which might affect the pedal ratio?

Your last master and booster had a hard pedal, but no issue with travel, correct? What you previously described with your old booster sounded like a vacuum issue or other defect with the booster.
 
898
544
Another thought, if your taper issue is the result of a bad wheel bearing, you will get pad knock back.

I had a front bearing go bad at Road America a couple seasons ago. The pad knock back was bad enough to cause the pedal to go to the floor in each brake zone. Fortunately, I had a spare hub with me.
 

captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
I'll be checking the bearings, but they're fairly fresh. I do get some mild knockback (but the 2-piece rotor is semi-fixed now, not all fasteners are sprung floaters).

The pedal assembly appears to remain unchanged (some sensor changes).

I've run 3 booster setups on the car so far:

  • Stock GT brembo-pack booster: longer throw, fairly soft
  • GT500 booster (13-14) and master: bad booster out of the box, rock hard and dangerous
  • Kohr booster (2008v6 model?): good actuation, but long throw and somewhat soft.

Driving around my S550 performance pack and its stock booster feels perfect. Its $100 plus whatever custom fabrication is needed to try it, I'm thinking about giving it a shot. Either that or I need to get serious about an autoblip to shave the time I'm giving up in footwork (and the wear/heat I'm putting in the clutch and transmission when I don't get it quite right)
 

captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
The S550 master cylinder will bolt to the S197 booster but the outlets are on opposite sides?? Is that what you are saying ??

Outlets and fastener locations are on opposite sides.

That said, the right hand drive mustang has the sides flipped (matching s197). Though I still don’t yet know the pedal geometry, bore stroke, and mounting geometry.
 
898
544
If you can figure out who makes your existing master cylinder and part number, you might be able to contact the manufacture’s support and see if it is also manufactured in a larger bore. It used to be common for a master to be built in multiple cylinder sizes to support different applications.
 
898
544
Another option to try is to drill another hole in the brake pedal for the brake pushrod below the current one. This will reduce the pedal ratio. It will firm up the pedal and might raise the pedal off the floor some. Raising the pedal would depend on the angle the pushrod has with the booster using the current pin hole.

If you try this, don’t make the angle on the pushrod extreme. You don’t want to damage the booster diaphragm. For the highest pedal, you need the pushrod to be as level as possible. Think the shortest distance between two points is a straight line.

Try drilling holes 1/2” lower at a time.

Older cars had brake pedals drilled for multiple pushrod locations. The mounting position used would depend on the brake calipers, master cylinder size, and if the car had power or manual brakes. The different holes are used to change the pedal ratio.

Update: I forgot to mention that the pushrod needs to maintain a 90 degree angle with the pedal when full depressed. Otherwise, you might create an over center issue with the pedal. You can usually work through this with an adjustable pushrod, but I don’t believe yours is adjustable.
 
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898
544
Here are some articles you should read with regard to pedal ratios and pedal / bell crank geometry. If you swap in an S550 booster and master, you will need to check and set these up properly. If not, the result can be a soft pedal, low pedal, or a hard pedal.

https://techtalk.mpbrakes.com/how-t...forget-the-booster-pin-to-master-cylinder-gap

https://techtalk.mpbrakes.com/bellcrank-classic-cars



Example of adjustable pushrod:


AAACABC2-5B5D-470E-BF34-51AFF7C830FA.jpeg

Unfortunately, there really aren’t any kits available for our cars that allow easy master cylinder or other substitutions. If you had an old car or hot rod, you would be in better shape.
 
The S197 and S550 brake boosters do not share the same mounting hole pattern. It would be difficult (although not impossible) to make the S550 PP booster and master mate up to your S197 dash panel. It would require a lot more fab work to make it work.
The S550 booster uses a 72mm pitch between all 4 studs in a square pattern. The S197 booster has a 68mm pitch between all four studs, but they are skewed a bit into a parralellogram (not square). Then, as has been brought up, getting the pushrod length and pedal ratios correct would also be troublesome. Overall, it looks to be way more work than it's worth.
While the bolt pitch is identical on the diagonals to be able to swap over the master cylinder from the S550 PP, the depth from the mating surface to the end of the MC piston are different. So that's a no-go as well.
Your issues with pad taper have more to do with the equal piston diameters for all 6 pistons (36,36,36). The reasoning for tapered pistons on a 6-piston caliper are to help with that. The 13/14 GT500 calipers have a tapered piston setup (30,35.5,39.5), and also are a lot closer (hydraulically) to the standard TP 4-piston (44,40) Caliper.
For S197 cars, I would probably stay with the 13/14 GT500 caliper that gets you 25% more effective pad area on the 15" rotor at the correct pedal travel and pressure. The S550 PP caliper nets you 30% more pad area, but with a compromised pedal and pad taper. Just my $0.02
 
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captdistraction

GrumpyRacer
1,954
1,698
Phoenix, Az
Good info, I hadn't been able to yet get measurements.

@OPMustang Tim , that definitely closes the doors on the hydraulic retrofit aspect.

That all said, outside the taper which I'll continue to track (possibly start rotating pads assuming rotor wear remaining even), I've gotten used to the throw and am relatively happy focusing efforts elsewhere to become faster.
 
Sorry to bring this thread back to life. Was reading through this and thinking of a solution, came up with this;

http://www.uprproducts.com/mustang-manual-strange-brake-kit-05.html

If willing to part with ABS, this might be a solution for those who dont mind a stiff pedal. It has a bore 1.125" matching the S550, obviously not vacuum assisted. Also maybe the brake line kit like the bellow would make routing of lines front and rear easier. A kit includes a proportioning valve to setup front and rear bias;

https://shop.watsonracing.com/MUSTANG-DRAG-RACE-BRAKE-LINE-KIT-p/wr-drbrklns.htm

I am not suggesting this as a solution, more like looking for peoples opinions on such a setup.
 

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