Question for those running CNC Dual Master Cylinders

[accobra66]

I am installing a dual CNC Master Cylinder with a balance bar.

I spoke to CNC today and they tell me that the distance from the pivot point of the brake pedal (where it attaches to the pedal box) to the push rod must be exactly 2".

I have the stock Mustang pedal box (from 90 Mustang) with the modified pedal (as per manual from FFR) and my length is someplace between 2 1/2" to 2 5/8". I can remove the pedal, have it shorted and that is fine.

They are telling me that with the length I have now I will not be able to develop the pedal pressure to get the 1,000 psi or so pressure at the calipers.

Has anyone else run into this? Does this change in length cause a binding interference in the angle that the push rod enters the M/C (or balance bar in this case)?

I could really use some opinions from those that have actually done this modification before I go hacking away and find that the angle does not allow the push rod as above to correctly enter the hole on the balance bar. That is the only thing I can think of here that would be a problem. Is there anything else I am not thinking of?

[SeanHM]

I'm not sure what the distance is on mine, but it's probably pretty close to yours. I modified the pedal like the manual explained and my pedal feels great. Night and day from the feel I had with the original master set up. The further the distance the harder you have to push to achieve the same pressure is the only real difference unless it puts you at an angle to bind as you mentioned.

[Arch F]

Delete suggestion, I didn't realize this was a completed car.
Good luck,
Arch

[accobra66]

Because removing a pedal box that is in a completed car I do not even think would be possible.

[MikeKelly]

I've been working on two of them lately - One was a retrofit to a Mustang II based kit from another manufacturer. The other is going on my Mk4 build.

I think the CNC unit is perfect for the application, but I don't think the people on the CNC tech line are familiar with what our specific application requires.

That said, 2" on the pedal is really close - what you need to accomplish is changing the (formerly power brake) pedal from about 3:1 leverage to somewhere between 6:1 and 7:1 leverage (needed for manual brakes).

In the Mustang II based pedal box, we originally had a 12" pedal with the pushrod mounted 4" from the pivot (3:1). We added a new mount for the pushrod at 2" from the pivot, mounted the new CNC unit 2" higher than the old booster / master cylinder assy, and it has worked very well. That car has never stopped like it does now.

In my Mk4, I'm working with a 94 pedal box and I'm pretty sure I'm going to end up moving the pivot point lower- I do not have room to mount the CNC unit 2" higher, so something else has got to move. It is a work in progress, I'm taking pictures for a "how to" post at a later date.

Anyway, the bottom line is you're going to need ~ 6.5:1 leverage, 2" puts you close to that. Once you have the leverage, you need to do whatever it takes to keep what you've moved from operating in a bind - I'm not sure what that requires on a 1990 pedal box.

HTH,

Mike

[MikeKelly]

Originally Posted by MikeKelly

I do not have room to mount the CNC unit 2" higher, so something else has got to move.

Just to clarify on this - It was easy in the Mustang II footbox to move the CNC unit 2" higher to keep the pushrod straight, so that was what we did.

I'm still working on mocking up the Mk4, the pushrod may turn out to be "straight enough". If not, the easiest answer for me is to change the location of the pivot.

There's a pretty sizable slot where the balance bar / pushrod assembly comes through the mounting base, so you *may* be alright on that.

I'll take some measurements when I get home and report back.

[cobramd]

I've been running a cnc for 5 or 6 yrs now. using the standard fox pedal box with the pedal mod as described in the build manual. I've never had any issues with not being able to lock my brakes. Well at least in the past the fronts have always been easily lockable the rears were a different issue which is due to caliper and weight issues. Now with wilwoods I can lock my fronts and rears at any time without issue.

[MikeKelly]

Ok, measurements taken - you could easily get into a binding issue on the pushrod angle. The good news is I *think* it's not too difficult to avoid, even on a retrofit (something I hadn't really been thinking about).

These are measurements off a CNC install on a Mk4 using a 94 pedal box (auto pedal box at that, manual pedal box is in transit).

I mounted the 94 pedal box in the holes FFR provided - perfect fit.

I mounted the CNC unit nearly horizontal side to side using the FFR provided hole on the drivers side (between the upper + lower pedal box bolts). I had to drill a new hole (in the Mk4) in the passenger side for the CNC unit. This new hole is just above (barely) an unused FFR provided hole (for the earlier pedal box?).

The CNC unit is mounted as straight and as high as possible without further modification of the FFR pedal box mounting plate.

Inserting a Craftsman 1/4" extension (which is a loose fit) in the back of the CNC balance bar (where the pushrod goes) - The closest you're going to get to the stock brake pedal pivot point without binding is about 3". You can't get to 6:1 like that.


Possible solutions (especially for retrofits)

1- I had to countersink the lower passenger side pedal box screw to get the CNC unit mounted flat without spacers. You could go up with the CNC unit about 3/8" before you have to countersink the upper pedal box mounting screws. Or, you could use spacers to mount the unit forward of the upper mounting screws. You probably need at least 1" up if you use this method, and you're going to have to open up (a lot) the FFR provided "center hole" for the master cylinder.

2- The pedal box mounting plate is about 10 degrees from vertical. You could mill the back of the CNC mount, or mill spacers to mount the master cylinders closer to level. Every little bit will help.

3- An offset push rod? I'm not sure that doesn't end up losing the leverage you're trying to gain?

4- The primo method (much easier for new construction) - move the pivot point closer (lower) to the master cylinder.

Any combination of these that gets you to not binding with 6:1 leverage should work pretty well.


Based on "feel" with the other retrofit install (and personal preference), I'm aiming for 7:1 (just for your information).

HTH,

Mike

[Bob Cowan]

I assume you're talking about this MC:




If so, there is another option. The CNC people are right, you need to have the correct pedal ratio for manual brakes - 6-7:1. Otherwise you would need a really strong leg to produce enough pedal power to perate the brakes at the limit.

Brake Article© by Dean Oshiro

Second, the push rod has to stay aligned with the MC. The usualy problem is that the pivot point moves in an arc as you depress the pedal. By simply shortening the brake pedal, the pivot point ends up at the very bottom of the arc, at 6 o'clock in a side view. As the point moves through it's arc, it only moves up. That's where you lose the alignment.

When you shorten the pedal, place a 7* dog leg just below the pivot point. That puts the pivot at about the 4 o'clock position when released. As it moves though it's arc, if firts moves down, and then back up. The up and down range of motion is cut in half.

You will need an adjustable push rod to set the pedal height. You might also need to modify the portion of the frame the pedal box is bolted to; you need a return stop that will prevent the push rod from falling out. I made mine adjustable, but once set I'v never needed to adjust it.


There's another reason for the dog leg mod. On a straight pedal, the pivit is around 6 o'clock. When you depress the pedel the pivot moves through the arc, and strarts moveing upwards. By the time the point hits 7-8 o'clock, there is almost no foreward movement of the pivit. It's only moving upward. No matter how hard you press, you get no more foreward movement of the push rod. The 7* dog leg ensure that the pivit point moves foreward through it's entire range. I got this tip from Richard Oben many years ago.

I set mine up this way from the beginning. I have never had a complaint about the stock brakes. They have always been excellent.

[CraigS]

When I installed my cnc's a few years ago i did the shortened pedal mod as described for manual brakes.Wasn't much fun on a completed car but done anyway.The brakes still required too much effort for me until I got Hawk HPS pads for the front and HP+ pads for the rear.Last mod i did was to drop the floor down to just 1/2 inch above the bottom of the 4 inch tubes.This allowed me to lengthen the bottom end of both clutch and brake pedals by maybe an inch.Helped a little too.

[MikeKelly]

Another good idea Bob, I forgot about the dog leg method.

Here is another post on that: Brake Pedal Modification

Another measurement - Looks like moving the pushrod mount point back 1" gains about 3/8" toward the pivot point on my mock up.

[letsgosteelers]

Hi Mike,

I bolted mine up today and it seems like both of these won't co-exist...is this the problem you ran into?

I have a Mk 3.1 - I thought I had seen a pic where someone had the four bolts in w/ washers and it was ok but you can see that lower bolt & washer cover up the hole

[Bob Cowan]

You have one bolt head in the way. Not a problem. Go down to Lowes and get another bolt with a head just like the ones that go through the MC mount.

If you can't find a bolt like that, get a button head, and fab a 1/4" spacer for MC mount, with a slight divit for the bolt head.

[letsgosteelers]

Hi Bob, thanks.

I took another pic which shows all the areas that seem to be blocked...I had never seen anything before that the 94' Pedal box needed to be modified, just always thought that it bolted right up.

I got a bolt similar to those on the CNC dual but the head extends out beyond the bolt hole and the two holes are so close together.

Bob, reading over your comments again I think the spacer idea might be the way to go, kinda like a half moon shape so that the bolt below for the pedal box can fit in there.

Kinda surprised noone else has run into this before or maybe its just everyone is using the pre-94 box w/ a CNC dual...

Mike, have you gotten any further on this?

[MikeKelly]

Sorry Steelers, I've been out of town - just got back home tonight.

Looks very similar, if not identical to the Mk4.

The master cylinder mount hole on the drivers' side I had to open up a little.

For the mc mount hole on the passengers' side I drilled a new hole just above the existing hole above the lower passenger side foot box mount bolt (now there are three holes right there). I drilled this hole to level out the master cylinders side to side.

I countersunk the lower passenger side footbox hole, and actually cut off part of the head of the countersunk bolt so I could mount the CNC unit flush against the mounting plate.

I'll try to post some photos in the morning.

Mike

[MikeKelly]

Let me start with compliments to Bob who did a very accurate job of explaining what I had done without actually seeing it, lol.

To avoid confusion about which pictures I'm writing about, I'll do several posts with limited attachments.

To clearance the bottom passenger side pedal box mount bolt, I countersunk the hole as far as I could without getting into the unused hole above it. This is in a Mk4 using a 1994 pedal box.

The Mk4 metal is not thick enough to countersink and get the bolt completely flush, so I used a lathe to cut part of the head of the screw off. The first two attachments are before + after pics of the bolt/screw.

Before anyone says I cut too much off the bolt, remember that there is very little stress on the bottom pedal box bolts. And, the passenger side MC bolt is going to capture that side of the pedal box too. This works for me, if you've got a better way - let's see it.


In the third attachment, you can see I had to enlarge the drivers' side MC mount hole (right side of image).

Also, I drilled a new hole in the passengers' side, just above an unused hole. I did this to mount the MC level side to side.

You can see the countersunk screw installed in the lower left just above the steering shaft.

[MikeKelly]

Here are some pics of the back side of the mounting.

I had to clearance the drivers' side (left side of picture) of the pedal box to get the nut + washer to fit flat (washer not installed in these pics). That hole just barely caught the edge of the 94 pedal box.

On the passengers side, the new hole for the MC captures the pedal box pretty well.

These pictures are taken using an automatic pedal box with the brake pedal removed so you can see the mounting detail. Hopefully, the manual pedal box is going in next week (it will have to be drilled, clearanced, painted, etc).

[MikeKelly]

And finally, here are some pics of the MC bolted up.

HTH,

Mike

[MikeKelly]

Attached is a pic of two Mustang brake pedals. The one on the left came out of a 1994 GT convertible w/ automatic trans. The one on the right came in from ebay, sold as part of a complete Mustang manual pedal box 1994 - 1998.

They both measure ~14" overall, but the auto trans pedal (left) measures ~3.75" from the pivot to the pushrod. This gives you an existing ratio of almost 4:1.

The manual trans pedal measures ~2.75" from pivot to pushrod. This gives you a ratio of ~5:1.


From page 55 of the Mk4 build manual (rev 3a) for the 94-04 pedal box (if used for manual brakes): measure down 1.5" from the stock pivot location and drill new holes for the new pivot location.

For the pedal : Cut 1.5" from between the pivot and pushrod and mig weld the pedal back together.


This is not a complaint about the build manual, just a heads up that you really need to understand what you're trying to accomplish (6-7:1 ratio).

Take 1.5" out of the pedal on the left and you get a ratio of ~7:1 (good to go).

Take 1.5" out of the pedal on the right and you get a ratio of ~10:1 (you just created scrap metal).


HTH,

Mike

[letsgosteelers]

Originally Posted by MikeKelly

And finally, here are some pics of the MC bolted up.

HTH,

Mike

Hey Mike,

Thanks for all the updates, looks great!!

I hadn't thought about leveling the holes but was wondering if they are placed at a slight angle for a reason? Does anyone know if that will affect the operation of the master cylinder?

In another thread, Bill (2Fast4u) built his own pedal box and the cylinders are also angled, not sure if that is for the lines to go in or for some other reason.

Ivan

[letsgosteelers]

Originally Posted by MikeKelly

Attached is a pic of two Mustang brake pedals. The one on the left came out of a 1994 GT convertible w/ automatic trans. The one on the right came in from ebay, sold as part of a complete Mustang manual pedal box 1994 - 1998.

They both measure ~14" overall, but the auto trans pedal (left) measures ~3.75" from the pivot to the pushrod. This gives you an existing ratio of almost 4:1.

The manual trans pedal measures ~2.75" from pivot to pushrod. This gives you a ratio of ~5:1.


From page 55 of the Mk4 build manual (rev 3a) for the 94-04 pedal box (if used for manual brakes): measure down 1.5" from the stock pivot location and drill new holes for the new pivot location.

For the pedal : Cut 1.5" from between the pivot and pushrod and mig weld the pedal back together.


This is not a complaint about the build manual, just a heads up that you really need to understand what you're trying to accomplish (6-7:1 ratio).

Take 1.5" out of the pedal on the left and you get a ratio of ~7:1 (good to go).

Take 1.5" out of the pedal on the right and you get a ratio of ~10:1 (you just created scrap metal).


HTH,

Mike

I had my pedal modded and came up with the following...unfortunatley didn't take measurements before the mod. Is that 5.5?

[MikeKelly]

Originally Posted by letsgosteelers

Is that 5.5?

Yes, that is ~5.5:1. Which may work fine with the Wilwoods depending on the amount of piston area for the calipers vs the bore size of your CNC master cylinders.

I've (with a lot of help from this forum and experience on someone else's retrofit) got my math + developement worked out for stock GT front and stock Thunderbird rear. I'm using 5/8" MC for the rear, 3/4" MC for the front. I'm going to need close to 7:1 to make that work right (jerk your eyeballs out stopping power, but doesn't hold up for racing as well as the Wilwood option).

I don't really care to hazard a guess as to what exactly your Wilwoods will require. 5.5 may be fine, I think if you plan ahead you should be able to keep some options open.

For instance, make sure that the pivot holes you use will allow some future pedal ratio modification (without binding the MC).

Make sure you leave yourself a way to get the pivot bolt out if you decide later that you need to change the pedal design.

Just a few thoughts,

Mike

P.S. I *think* the extra holes are for the earlier pedal box (using the earlier pedal box probably leaves some of the 94-04 holes unused). I have seen the CNC MC installed on the slant (probably using existing holes), and they seemed to work alright. I wanted mine mounted level side to side.

Also, mounting on the slant is going to lower the pushrod entry vs. the pivot point, and we're already lowering the pivot point to avoid binding...

[snakeboost]

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[nc1mike]

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