Master Cylinder Question

[mike w]

I recently purchased a completed MKIII from the original owner/builder and I am not happy with the amount of effort reqired to stop the car. After some investigation I have found 3 things that I would like to address. One is the master cylinder. I have read that the 94-95 Mustang Cobra with the 15/16" bore is recommended. I also read on another forum that they used a 1-1/8" bore master cylinder from Master Power Brakes. Does anyone have any experience with this master cylinder?

http://www.mpbrakes.com/products/product-detail.cfm?product_id=526

The second item that I would like to address is the brake pedal modification. The brake pedal hinge bolt is still in the power brake position. It looks like it will be difficult trying to get this bolt out to drop the brake pedal without performing some type of surgery to the drivers footbox. I thought by installing the 1-1/8" bore master cylinder first that I may not need to modify the brake pedal. The third item that I would like to address is the need for an adjustable proportioning valve for the rear brakes. I see the FF recommend installing one for the rear brakes. Can this be installed next to the master cylinder or do i need to install it near the rear end just before the tee to both brake lines?

[riptide motorsport]

the smaller the bore of the master cylinder the more "power" it will give you. Power asst. brakess have a larger bore, manual btakes have a smaller bore master cylinder....think of a volume of water in a 1/2 onch hose, now put that same volume through a 1/4 hose........you get more pressure. use a maual master cylinder......HTH ,,,,,,, , Steven

You can put the brake bias (proportioning) valve right after the master cyl. should you care to.

[Jacob McCrea]

I'll take a crack at answering one of your questions. My understanding, albeit based only on research and speaking with other builders, is that you need to find the master cylinder diameter that best matches the piston area of your calipers. As for the pedal, my understanding is that you really, really should do the pedal mod to increase the leverage. Can't say I've ever driven one (I'm still building), but I understand the difference is akin to riding a motorcycle and grabbing the front brake lever near the pivot vs. at the outer edge of the handlebar. The greater leverage from the latter gives more brake pressure for a given amount of effort.

There are others here who understand all of this much better than I do. Without knowing what calipers you have I am not sure if you'll be able to get very specific advice re: what master cylinder does or does not work. Also, for what it's worth, there is a good thread on the other forum titled "Good Brakes on a Budget," under the coupe sub-forum, which addresses a lot of the issues that arise with incorporating a Mustang-based braking system into these cars. Good luck, and welcome to the forum.

[1fastsedan]

To start with, here is the troubleshooting grid from Wilwood. http://www.wilwood.com/TechTip/TechTroubleshooting.aspx

Here is some good reading on pedal leverage (http://www.wilwood.com/TechTip/TechPedalTip.aspx), but yes you should modify your pedal's point of rotation if not using power brakes.

[oldguy668]

PM Mike Everson and he'll tell you how to get that bolt out and back in. In addition, once the bolt is out you'll have to modify the pedal (Mike will do it for you). You also need any MC with a 15/16 bore. If you do those three things your brakes will work great. You are giving up mechanical advantage with both the oversize MC and the bad pedal location.

[AC Bill]

Another common master cylinder is the 15/16" Jeep one, as it has a larger diameter as well. You can get a cast aluminum one that is quite lightweight. If you have rear disc brakes especially, I believe the larger diameter m/cylinders are required to be able to move the needed fluid. I would do the pedal mod for sure! With the right pads and these other mods, your braking effort should be substantially reduced..Still not as easy as power brakes..but heck this is a replica of a 60's sports car..:)

[mike w]

Thanks to everyone for your responses to my questions. You confirmed what I was already thinking which is that I will have to make the brake pedal modification. I already visited Mike Everson's web site and am planning to send it him once I get it out. I will change the master cylinder to the 94-95 Cobra 15/16" bore since I have disc brakes on all four corners. I also plan to install the proportioning valve on the rear brakes so that I will be able to dial in the front/rear braking bias. Once again thanks for you input.

[AC Bill]

When running the adjustable prop. valve on the rear line, almost all builders that used them, said they needed to run them wide open, so it was in fact not playing a roll anyways..The roadster tends to lock up the fronts first, so reducing the rear pressure is pointless. Some have even gone as far as to add the valve to the front, to reduce pressure to them, so it equals out better with the rear..( even several spec racers do this). Although somewhat controversial, it seems to work.

Many don't use any valve at all and have simply played around with different brake pad compounds to get effective, and more equal braking.
Hawk HP Plus pads on the rear calipers, and Hawk HPS pads on the front, seem to offer the balance needed. They do need to be broken in properly, as per Hawks website instructions. They are also dusty compared to other compounds, so expect to be cleaning your wheels more often..I think Porterfield R4S pad has less dust, and works as effectively as well, but man are they expensive..:(

Here is a lengthy thread that is really worth reading through, to help you decide which route you may want to go. The discussion goes way beyond what the original poster asks, so read completely through it.

http://www.ffcars.com/forums/17-factory-five-roadsters/245120-brake-bias-concern-warning-safety.html

[Bob Cowan]

When running the adjustable prop. valve on the rear line, almost all builders that used them, said they needed to run them wide open, so it was in fact not playing a roll anyways..

This is very true. The "problem" with a cobra is that you're using a brake system designed for 60/40 weight bias on a car that's more like 48/52. You generally need a lot more rear brake bias than a stock set up can give you.

The brake porportioning valve that most people use is not really a proportioning valve - it doesn't proportion anything. It's a pressure reduction valve. It's used to reduce the pressure in that hydraulic system. Since there isn't enough pressure to begin with, why would you do that? Also, with the valve all the way open, it still reduces pressure by 8-10%. Some people have put this valve in the front brake line with good results.

The only way I know of to get the perfect balance is to use two master cylinders with a balance bar. If you're going to be racing, spend the bux for this to go with your giant Wilwood brakes. Well worth it.

If the stock brake pedal is in the power brakes hole and has not been shortened, it doesn't matter which MC you use, they will never work right. Fix that before doing anything else, before spending another penny on any other component. If you want the absolute best braking with a stock type MC, here's what you need to do:

1. Shorten the pedal to creat the correct ratio
2. Add the dog leg to improve the piston motion
3. Use an adjustable push rod AND an adjustable return stop.

It's a little bit of work, but very little money.

[mike w]

Bill & Bob, thanks for your added input. Even though it will be a difficult task getting the brake pedal out with everything assembled, that is what I plan to do first. After it is modified I will reinstall it with the adjustable push rod. After I test drive it then I will decide if I neet to change the master cylinder to the 15/16" bore. As for the proportional valve, thanks for your advise.

Bob, I have one question, what did you mean by the "dog leg"?

[Bob Cowan]

Bob, I have one question, what did you mean by the "dog leg"?

Wish I could take credit for this, but it wasn't. Richard Oben at North Race Cars came up with this one. I did it to mine, and I'v always had excellent manual brakes.

http://www.norcal-cobras.com/projects/brakes/brake_pedal.htm

[AC Bill]

I never saw those modifications of David Borden's before, interesting...

I see he mentions he didn't shorten the new brake pedal he was using, but I believe that pedal, perhaps from the earlier 4 cyl Mustang he was using the master cylinder from, was already shorter one to begin with. That was an alternative pedal that could be used instead, to avoid cutting, and welding the later model PB pedal, as many buileers needed to.

In my case (90 GT donor) I followed the FFR manual, as to how much to cut out, and where to cut it from. I had a professional welder do it all for me, I simply marker where to cut it. With something as important as a brake pedal, I didn't trust my welding skills..(only cost $20 to have him do it).As well, I simply purchased the adjustable rod, from Breeze, rather than make one. It works great, is inexpensive, and really helps on getting it all set up so the brake light switch works effectively. I didn't add an adjustable stop, simply a rubber cushion on the cross frame. Perhaps with certain year/model donor pedal boxes this is needed?

Not sure what year pedal box you are using, but the earlier ones, 87-93, I believe, already had existing holes in place, to be able to lower the pedal arm pivot point. I gather that this was because the same pedal box was used on non-power brake Mustangs, and Ford would just use the lower hole for them. Later models boxes need this hole to be drilled.

You may also want to tweak both brake and clutch pedal arms, while your involved in this job. This tweak involves putting a "sideways" dogleg in both pedal arms, to move them slightly to the left. This allows a little more shoe clearance between the brake and the gas pedal. It can prevent you stepping on both at the same time, while braking..
This is a different dog leg than David Borden describes in his modification. I don't really understand what that accomplished, other than perhaps lowering the pedal closer to the floor slightly? He doesn't really discuss that modification fully, as to what the point was?.. I never did that on mine, and it works just fine.

One thing I might mention, (which I have still to finalize on mine..lol) is to try and get the throttle and brake pedal set up, so that when the brake pedal is depressed, (brake full on), that you can heel/toe, the throttle pedal. This will make for much more comfortable starts, when your stopped on a steep hill..(believe me..:) ), and perhaps for spirited driving on a windy road. It may take a little adjusting on the throttle cable, and throttle pedal mounting..

[Bob Cowan]

The dog leg mod is a little difficult to describe, so I drew this quick scetch to (hopefully) make it less muddy:

http://i13.photobucket.com/albums/a256/bobcowan/Wilwood%20Brakes/DogLeg.jpg

With a straight pedal mounted in the stock box, the arm is almost vertical. At rest, the push rod pin starts out around 5 o'clock. As you depress the pedal the pin moves in an arc. When you get about 1/2 way through it's travel, the pin is now moving up in the arc. That gives it very little foreward motion for a lot of pedal effort. Very inneficient. And some MC's can't manage that much up and down movement without binding

With a 7* dog leg below the pin, the pad is in the sam position. But the pin has moved to the rear, closer to the driver. At rest the pin is higher up on the curve. As you depress the pedal, the pin moves through it's arc as before. But there is less upward movement and more foreward movement. The brakes are very easy to modulate, and very linear.

Like Bill did, I took my brake pedal to a welding shop and had a pro do it.

Now that the top of brake pedal is closer to the driver, it didn't fit my MII pedal box very well. I cut out the cross bar and made it adjustable. That along with the adjustable push rod made it real easy to get the brake pedal exactly where I wanted it.

http://i13.photobucket.com/albums/a256/bobcowan/Wilwood%20Brakes/smallpedalstop.jpg

[mike w]

Bob, this sounds very interesting, but I did not see the sketch that you mentioned. As for drilling new holes in the pedal box, that will be near impossible for me without disassembling the entire foot box at this point. I am wondering how this dogleg would work in the lower set of holes for the manual brake position. Any thoughts?

[AC Bill]

Mike, I agree, pretty sure you will have to pull the pedal box to drill it..I don't think you could squeeze an angle drill in there on the throttle pedal side..:(

Bob, thanks for dog leg explanation..:)

[Bob Cowan]

I included a link to that pedal box picture because it was the only one I knew of that shows both holes. The top hole (empty) is for power brakes. The bottom hole (also empty) is for manual brakes. After shortening the pedal for the proper ratio, use the bottom hole.

You can find the sketch on my Photobucket account. Do a search for "bobcowan", and look in the Wilwood album.

[CraigS]

To somewhat recap all that has been posted I would recommend you do it in this order.1-Mod the pedal and get that working correctly as it is the basis for everything else.On both my MkI and MKII, the head of the bolt was toward the left fender. So I holesawed a hole (not fun at an angle but doable if careful) in the footbaox alum panel that let me pull the bolt out of the pedal by putting the head thru the alum far enough that it came out of the pedal box and then back inside the alum at an angle to remove it. 2- the pedal will help a bunch but the smaller MC will be another big improvement. 3- hawk pads. Try HPS in the front and HP+ in the rear. I have been thru all this on two cars and the above is my experience. I have a dual MC now and the above pads are perfect but I do have the advantage of being able to make adjustments to front/rear bias.So I can't promise the pads are perfect for you but they will be close. If there is too much rear brake you can always do a pressure regulator but that should be a last resort.Another thought. You may think about power brakes but remember this. PB will get the required effort into a better range but will do nothing for the needed increase in rear brake (compared to the mustang your brakes came from).HTH

[mike w]

Craig, thanks for your advice. I plan on doing the pedal mod first. The head is on the clutch quadrand side so I will need to hole saw that side of the foot box to get it out. It looks like I will have to reinstall it from the opposite side in the lower holes since the clutch quadrant will interfere. That will require another holesaw opening. Once that is done, then I can address the brake pad and MC issues. I am not sure that I understand your comment about the "smaller MC" My current MC is from a Mustang 5.0 which is .82" bore. Most recommendations including FF for cars with 4 wheel discs is to go to a larger MC from the 94-95 Cobra with a 15/16" or .9375" bore. This would be larger, not smaller. Wish me luck!

[AC Bill]

Most recommendations including FF for cars with 4 wheel discs is to go to a larger MC from the 94-95 Cobra with a 15/16" or .9375" bore. This would be larger, not smaller.

Exactly..you will need a larger bore M/C to push more fluid volume, to move the rear calipers effectively. Wheel cylinders on the drum brake cars, didn't require the larger volume of fluid to work.

[CraigS]

A smaller MC multiplies effort at the expense of more needed travel. As a comparison, I have 3/4 front and 5/8 rear MC in my CNC dual MC setup. Not sure how fluid capacity of these individual MCs compares to a normal MC but mine works great.RE; 2 holes sawed. It's been about 3 yrs since I did the pedal mod so my memory is a little hazy but I am pretty sure I installed the bolt in the lower holes w/ the head to the left. I may have had to move the clutch pedal or remove the quadrant to get it in but I definately made only one holesaw cut.HTH