Manual brake tech questions?

[KAILUAZ]

Yes I used "search" but no one seems to be running this particular setup.
So here is what I am looking to run on my 86 (former 4 cyl). I plan on having the car weighing in just under 3000 lbs. This will be a track/autocross car with some street driving to and from events etc.
Heres my plan:
-PBR front calipers on stock rotors
-LSC rear end (54mm pistons on 11.35" rotors)
-Not sure what pads to run yet
-MM manual brake conversion kit
-Gutted stock proportioning valve
-Adjustable proportioning valve
-SS lines up front
-New hard lines front to rear
-Not sure what size master cylinder size to use (1" or 15/16") Might go with the Town Car M/C.

Now, over at Corner Carvers there is a thread about pedal ratios etc, but its way to technical for me (I hate math). Anyone have a educated opinion on this setup?
Thanks

[Jack Hidley]

Your rear brake calipers are way too large. With a 54mm piston, they have exactly double the surface area that the stock Varga 38mm caliper or the PBR 38mm rear caliper has. That means that the SVO rear brakes will have double the brake torque for a given hydraulic pressure than the stock Varga rear brakes will have. The rear brakes will lock up very early. You can't fix this with the proportioning valve, it is too large of a mismatch.

For master cylinders a 1" 93 Cobra model will work fine with the dual piston front PBRs and 13" rotors.

[KAILUAZ]

Jack any ideas on a good setup? I'm stuck with the rear end, but can do something different with the fronts, as I haven't bought the calipers yet.

[Jack Hidley]

Unfortunately, there is no front brake option for the SVO rear disc, other than the SVO front brakes (73mm calipers with 11" rotor). Running all SVO brakes on the car will give you a proper brake balance, but they aren't really suited to track use since the front rotors are the same size as the 87-93 stock rotors. They have very little thermal capacity.

Also the brake pad selection for the front and rear SVO brakes is very limited.

To balance the SVO rear brakes with a 13" front brake, you are going to need a four piston front caliper with pistons sizes of about 52mm. This is huge. I've never seen anything this large.

I know you probably don't want to hear this, but it's the truth, if you are going to use the car in any situation where you will be using the brakes to their maximum capability (Track, AutoX or panic stops on the street), your cheapest solution is th get rid of the rear brakes. Find someone with Baer PBR calipers, 12" rotors or any 94 and up Cobra rear brakes (Varga caliper and 11.65" rotor). In either case, you will also need the mounting brackets for the calipers and the axles.

I assumed that PBR calipers on stock rotors, means dual piston aluminum floating PBR calipers (Corvette/Cobra type) on a stock 13" Cobra rotor.

[KAILUAZ]

I assumed that PBR calipers on stock rotors, means dual piston aluminum floating PBR calipers (Corvette/Cobra type) on a stock 13" Cobra rotor.

Its the stock 99+ Mustang GT/V6 brake calipers and rotors.

Anyone know how hard it would be to refit the 99+ PBR rear brakes/brackets onto this LSC rear end?

[shaqer74]

I know a local guy here is running the 99+ V6 calipers and LSC rear brakes without a problem. This setup is on an '86 GT with SVO MC and stock booster. He loves it. I am working on installing this exact setup in my '86. The key is that you need the larger MC to supply enough flow to the large calipers. BTW...his car is being used for racing in the SCCA CP class and does quite well.

Calculations...stock SVO is 73mm front and 54mm rear = piston area 9.62 square inches

99+ PBR front and 54mm rear = piston area 7.95 inches

check my calculations here (bottom of page)...http://www.factoryfive.com/table/wha...akeadvice.html

I understand what Jack is trying to say, but IMHO this does seem to be a better setup than factory SVO. My recommendation is too go ahead with your plans, but use a larger diameter MC.

[Jack Hidley]

Here are some numbers to explain what I'm talking about:

Caliper diameters in mm Area in sq in
SVO rear 54 3.55
SVO front 73 6.49
Varga/PBR rear 38 1.76
PBR/Cobra dual 38 x 2 3.51
PBR GT/V6 99+ dual 44.5 x 2 4.83
94-98 GT/V6 single 66 5.30

In a stock Mustang with an inline master cylinder setup, there are two master cylinders, but since they are inline and have the exact same piston area (they share the same bore) when you press on the brake pedal, you are going to get exactly the same pressure in both the front and rear hydraulic systems. Therefore the master cylinder can have no effect on the front to rear brake balance. It only effects the amount of pedal travel and effort required to stop the car. Once you have selected calipers, you select the master cylinder to get the correct effort and pedal travel that you want.

The primary thing that effects the front to rear brake balance in the car is the ratio of front to rear brake caliper piston areas. The ratio of front to rear brake rotor diameters affects it also, but since this ratio only varies about +/-4% in this case, I'm going to ignore it to simplify the discussion.

A stock SVO has a front to rear area ratio of 1.83 (6.49/3.55). For comparison a 99 Mustang has an area ratio of 2.74 (4.83/1.76). A 96 Cobra has an area ratio of 1.99 (3.51/1.76). The proposed brake system under question (99+ PBR dual front and SVO 54mm rear) has an area ratio of 1.36 (4.83/3.55). This is a huge change in the direction of increased rear brake bias. Compared to a 96 Cobra, it is 30% (1-(1.36/1.99))*100 increase in rear brake bias. This is huge.

You will have to use front pads with the highest coefficient of friction that you can find. Rear pads with the lowest coefficient of friction you can find. Install an adjustable proportioning valve and run it turned almost all the way down. Doing all of this, the car will probably still lock the rear brakes early and will have no tuning ability with the proportioning valve available. You'd be better off using the 94-98 front calipers. This will give you an area ratio of 1.49 (5.30/3.55). This will only be a 25% (1-(1.49/1.99))*100 increase in rear brake bias. Not a huge improvement, but some.

I'm not sure what is going on with shaqer74's friend running a Mustang in CP. My guess is that he is running a front race pad with a very high cf and a rear pad with a very low cf. In CP you are allowed to add a couple hundred pounds of balast to the car. People typically build the cars quite a bit under the weight limit and then add balast where they want it. Balast will always be added to the rear of the car to increase weight over the rear tires. Doing this will require an increase in rear brake bias over a stock Mustang to get maximum braking.

[shaqer74]

Originally Posted by KAILUAZ

Anyone know how hard it would be to refit the 99+ PBR rear brakes/brackets onto this LSC rear end?

The 99+ rear brakes are almost a direct bolt on, but you will need a different axle setup. You will need the 94-98 axles (.75" wider per side compared to stock) or north racecar brackets and stock length axles. Note, the LSC rear is 1.25" wider per side compared to stock.

[guywwagner]

While we are on a manual brake thread I have a question.I have a 95R road race car and I'd like to convert to manual brakes.It has PBR calipers front and rear with 12" rotors at both ends.MM does not make a kit,but I see that steeda makes an adaptor to eliminate the booster.Will this work or will the pedal effort be too high? The car weighs 3100 with fuel and driver.I'm using Hawk blues now but I'm changing to DTC 70/DTC 60 pads.
Thanks, Guy

[Jack Hidley]

Guy,

A stock Fox Mustang with power brakes has a mechanical pedal ratio of 3.4:1. The stock manual brake pedal has a ratio of 6.1:1. If you remove your booster, you brake pedal force is going to up by at least 80%. There is no brake pad out there that has a cf that is 1.8 times that of a Hawk Blue. That is what you would need to maintain your current brake pedal effort.

I assume that this is an A Sedan car? The front brakes are PBR Cobra calipers (38mm pistons)? What are the rear brakes?

The HT-10 has a higher cf than the DTC60 or 70. If you are going to try to convert to manual brakes, I'd use that pad first. Two other good manual brake pads are Wilwood H compound and Pagid RS15.

[guywwagner]

Jack,
My rears are a Baer/PBR setup.They are the same caliper as a 93-97 Camaro. Are the pedal ratios the same on my 95 as a Fox? What I'm looking for is a pedal that requires more force but nothing crazy.I find it difficult to threshold brake with the amount of boost my system currently has.
Thanks,Guy

[Jack Hidley]

Guy,

Off the top of my head, I don't remember if the SN95 power brake pedal ratio is the same as that of the Fox cars. I just mentioned the Fox ratios as a comparison of manual to power ratios in the same car. Let me look around and see if I have the SN95 pedal ratio.

If your car has a 1" 95R master cylinder, then changing this to an 87-93 master cylinder will give you about a 32% increase in travel, which will give you this much more leverage. If you combine this with the H compound Wilwood or Hawk HT10 brake pads and raise the brake pedal 1-1.5", you might be ok.

[Oktavius]

Originally Posted by Jack Hidley

Guy,

Off the top of my head, I don't remember if the SN95 power brake pedal ratio is the same as that of the Fox cars. I just mentioned the Fox ratios as a comparison of manual to power ratios in the same car. Let me look around and see if I have the SN95 pedal ratio.

If your car has a 1" 95R master cylinder, then changing this to an 87-93 master cylinder will give you about a 32% increase in travel, which will give you this much more leverage. If you combine this with the H compound Wilwood or Hawk HT10 brake pads and raise the brake pedal 1-1.5", you might be ok.

Do you know what the pedal ratio would be with your manual brake kit?

[Jack Hidley]

Tim,

The pedal ratio on the MM kit is adjustable from 5.7:1 to 6.5:1

[Flyin5-o]

Jack, I would think all this area ratio stuff apply to power brakes also? If so it sounds as if my current set-up is gonna work like crap.
front :'87-93 stock fronts, EBC pads, braided lines,caliper sleeves
Rear: '82 camaro front GM metric caliper (60MM piston I think) w/ turbo coupe discs

'87-93 m/c, gutted stock prop. valve ,and adjutable prop. valve.

I haven't driven this much but it stopped OK the couple of times I drove it to storage(I never have stopped hard w/ it yet) .

[Wrencher]

Hi there Jack, you seem to really know your stuff! Let me pick your brain a little.

I'm building a 473" Big Block '84 Hatchback. I currently have '96 GT front brakes (dual piston PBR's) and I'm getting a rear end and brakes from a '97 GT. Not sure on the specs there. I'll be going with manual brakes as well, and I really suck at math.

I have a Wilwood adjustable prop valve if I need it, but this car is 100% gutted and I'm starting from scratch.

What master cylinder will I need and how do you recommend I run the lines to the rear? Basically I'm building the whole thing from scratch.

Any help would be greatly appreciated.

[Jack Hidley]

Flyin,

If your rear calipers really have 60mm pistons in them, then yes, be prepared for the car to swap ends when you have to stop it hard. Once you get the car running, test the brakes on a wide desserted road with tires that you don't care much about.

[KAILUAZ]

Does anyone know if I can swap the 99+ PBR calipers on the LSC rear end? I'm sure the brackets are different, how about the spacing for the rotor on the LSC axles?
Or should I just try to find a 99+ rear end? I don't care about them being wider like the LSC.
Thanks!

[shaqer74]

I finished my 5-lug rear disc upgrade recently and after adjusting and breaking in I took it racing at an SCCA event. So now I thought I should give an update for others wanting to know is it true about the SVO rear brake lockup issue with anything other than SVO fronts. In short, it is true the rears do lock up before the fronts after completely adjusting the prop valve out. However the lockup in my case is only at low speeds on new blacktop, the SCCA event was on concrete and I experienced very good feedback from the brakes with no lockup at all. Here is my setup...Front, 99+ PBR calipers, Performance Friction pads, Decelarotors. Rear SVO calipers, Bendix Ceramic pads, Decelarotors.

Overall I am very pleased with the setup, the only time I experienced lockup was on fresh smooth blacktop while adjusting the valve. Never any rear lockup on concrete. Now I know that if I ever drive in the rain or on wet pavement, I may experience lockup, so I am checking into options to further reduce rear bias. If you have any ideas on how to do this, please let me know...Jeremy

[KAILUAZ]

Jeremy, thanks for the update. Did you gut the stock proportioning valve?

[shaqer74]

Did you gut the stock proportioning valve?

yes I did.

[Quik_Stang]

Hey Jack, I currently have a 1993 4cyl mustang. I have bought and installed a 5 lug conversion with the 03/04 Cobra style brakes front and rear. I got all the necessary conversion lines from MM to install a 15/16" 94-95 SVT MC I have on the stock 4cyl booster and I was also going to run the Wilwood PV I got from MM also. Not knowing how this booster MC combo is going to work with the 4cyl in it, and then how it will work later when I put in a 351w with a large cam in it, I was thinking of going to manual brakes(via your guys conversion kit). The car will be driven as a weekend/street car, not a DD. So my question is, with the above info taken into account, would you recommend manual brakes for the street? And if not, what MC/booster combo would you recommend for the current 4cyl motor that would still work when the 351w w/cam is dropped in? Thanks a lot for your help! Brent.

[2ndsideways]

So stock is 3.4:1 and what sounds like the better adjustment of the MM kit is 5.7:1, this works out to around 55% harder pedal than stock?

Im suppose to buy this kit monday and this thread is making me think twice.

Im gonna start researching this more.

I plan on using the 2000 Cobra R 13" 4 piston Brembo kit, and the 11.65" Cobra rear Varga caliper kit which was also on the 2000 R. This should all work well minus the booster? I have a 15/16"ths MC for this.

[Jack Hidley]

Brent,

If you have a street car and don't have a strong need or desire to install manual brakes, I would keep the power brake system. I don't think there will be any problem with getting your brake system to work with both engines.

The basic front to rear balance of your brake system should be fine. Once you install the adjustable proportioning valve, you will need to optimize the balance differently for each of the engines.

Your main problem with the 351 will be low vacuum and lack of space. From the lack of space standpoint, I would keep your existing brake booster which is smaller than the SN95 8" models. It will give you the most clearance to the taller deck 351 heads. If the 351 doesn't have much manifold vacuum, you can add a vacuum reservoir to the car. Moroso and others sell these. It is basically just a medium sized plastic or metal drum with an inlet for a hose. You remove the check valve from the inlet of your existing booster. Connect the booster and vacuum reservoir directly together with a tee fitting. Put a new straight shaped check valve on the third leg of the tee fitting going back to the intake manifold. This will increase the average vacuum level at the power booster for more consistent brake pedal effort.

Ryan,

Your missing one important thing in your assumption.

The brake pedal ratio is the output to input force for the brake pedal. So if the ratio is 3.4:1 and you apply 100lbs of force on the brake pedal, there is 340lbs of force applied to the pushrod. With a power brake system, the power booster will then increase the input force by a factor of around 2. So the 340lbs of force on the input of the booster becomes 680lbs of force on the outlet of the booster which is applied to the m/c input.

With a manual brake pedal that has a 5.7:1 ratio, with 100lbs of input force, you get 570lbs of force applied directly to the input of the m/c.

I've over simplified the behavior of the booster here, but the general point is that both systems will generate about the same force levels on the input of the m/c. The small difference that you usually have is compensated for in a manual brake system with a slightly smaller m/c diameter, the achieve the same hydraulic system pressures. The 15/16" m/c will work fine with that brake setup.

[silver408coupe]

How much vacuum is needed for proper operation of power brakes? My cam card says my cam should make about 12in. at idle.

[Quik_Stang]

Thanks a million Jack.

[2ndsideways]

Sounds good Jack, still wish I could just drive a car with the MM kit in it to feel the diff for myself.

[Jack Hidley]

There is no particular amount of vacuum pressure that is required to operate a vacuum powered power booster.

The booster operates by having atmospheric air pressure applied on one side of a piston and vacuum on the other side. This creates a force which is a function of the area of the piston and the pressure of the vacuum (relative to atmospheric). If your engine generates 12" of vacuum (5.9lbs/sq in) and the booster has a piston in it that is 6" in diameter (28.3 square in), then the force on the piston is 166lbs (5.9lbs/sq in*28.3sq/in). Note that this is the maximum force the booster can generate. Exactly how much force the booster generates at any given instant is a function of how far its input pushrod is pushed, as this regulates how much air is let into the chamber on one side of the piston. The farther the brake pedal pushes it, the more force the booster adds, up to the boosters maximum force for the amount of vacuum pressure it has available.

Everytime you release the brake pedal, you use up some of the vacuum volume in the booster by replacing it with air at atmospheric air pressure. Everytime this happens, the vacuum pressure level in the booster drops, until eventually the air is at atmospheric pressure and the booster can no longer supply any assist. In an OEM engine, the engine generates enough vacuum pressure and volume to keep the brake booster at a constant pressure level, assuming that the brakes aren't used off and on constantly while the engine is held at WOT.

With your engine, there is enough vacuum pressure to make the brakes work (but the brake pedal will require more effort since the vacuum pressure is lower). I think the main problem is that the engine won't make enough vacuum volume to keep the vacuum pressure level in the booster constant. The brakes may use the vacuum up faster than the engine can replace it. In those situations, the brakes are going to go from feeling a little stiff to really stiff. The solution to this is a vacuum reservoir to help keep the vacuum pressure level for the booster constant.

[silver408coupe]

Jack,

Thanks that is a great explanation. I could see it in my head finally. I am running manual brakes but was wondering if the cam could allow me to switch back to power if I, for some reason, didn't like the manual feel. I am going to run:

Front-Baer 14" 2 piece rotors w/6 piston calipers (6S) and braided lines.
Rear-Baer 13" solid rotors with 95' calipers and braided lines.

The rest of it will be-Maximum Motorsports manual pedal kit, gutted stock prop valve and Wilwood adj. prop valve. The master cyl. I am unsure of the size. I think I purchased it as a mid 80's Ranger with manual brakes. It's probably going to be 1" or bigger.

I will work with the pads once it stops well to max it out. This won't be something to be competitive in, just something to have fun in. But I do expect quite a bit from it.

Thanks for all your help, you teach us noobs quite a bit here.

[2ndsideways]

So like I stated earlier in this thread-- "I wish I could feel the MM manual set up to know for sure", I needed a worst case scenario to make a descision on if this kit was for me. I remembered while driving my daily driver/ parts car '91 4 cylinder hatchback (stole black interior from & fenders, doors) that I could get up to 65mph or so and put it in nuetral and turn it off- pump brakes lightly to shed off remaining vaccum and then try and panic stop. Much to my surprise I could stop quite quickly and even lock up the tires, so now knowing this and the amount of force I needed to apply to do so I feel the MM kit will/ should be perfect for me. The MM kit wont be anywhere near the amount of pedal stiffness at its minimum adjustment, much less using all of its available leverage/ adjustment.

I ordered it Monday to use with a 15/16"ths mc that I had new. I also got a TCI line lock since I am re-plumbing the whole car.

This is quite a nice write-up on the kit: http://www.stangtv.com/forum/maximum...1-a-5059.html#

[Jack Hidley]

Ryan,

If you don't have a problem stopping a Mustang with power brakes without the booster operating, you will have no problem with manual brakes.

With regards to that StangTV project car. Since you are also installing a line lock, make sure you plumb the system the same as they did in the article. The line lock must be placed after the stock proportioning/combination valve. If you place it between the m/c and the stock proportioning/combination valve, once you release the line lock, the rear brakes could stay locked.

[2ndsideways]

Originally Posted by Jack Hidley

Ryan,

If you don't have a problem stopping a Mustang with power brakes without the booster operating, you will have no problem with manual brakes.

With regards to that StangTV project car. Since you are also installing a line lock, make sure you plumb the system the same as they did in the article. The line lock must be placed after the stock proportioning/combination valve. If you place it between the m/c and the stock proportioning/combination valve, once you release the line lock, the rear brakes could stay locked.

Hey Jack,

I planned on re-plumbing the entire system and starting from scratch, therefore eliminating the stock prop valve, adding a willwood adjustable unit which would be inside the car.