Brake pedal modification for increased travel.

jeepjoint · Oct 26, 2011

I've searched a don't believe I've see this addresed.

I have a 1955 2DR post with 4W disc brakes. A new M/C with 1" piston & a CPP 7" dual diaphragm booster.

Most of the posts that deal with increased pedal stroke/travel suggest drilling the hole for the booster rod 1" below the original hole for manual brakes. In my case, it seems like it would put the rod between the pedal & booster in a slight bind.

I called CPP and asked their opinion and they agreed to drilling the attachment hole 1" lower on the pedal to increase stroke. But they also said I needed their bracket #5564BB to mount the Booster & M/C at an angle that corrects the pushrod geometry back to a straight line between the pedal & booster.

This is the first that I've heard about this new bracket requirement. QUESTION: Are those of you that have the booster mounted perpendicular to the firewall & the pushrod mounted 1" lower than original on the brake pedal experiencing any problems? Some of the posts suggest a "Z" pushrod that I haven't been able to locate as yet.

My main concern with CPP's bracket is that the front reservoir will be somewhat higher than the rear. CPP did say it wasn't as radical as the Camero's, but still elevated.

Any comments are welcome.

595544 · Oct 26, 2011

You can put a couple of flat washers on the 2 lower studs, between the firewall and booster brackets, that will give you a slight upward tilt and allow for the rear rod to align with the 1" lower hole in the pedal. This will also bring your pedal travel up about an 1" to 1.5' higher then it is now.

Rick_L · Oct 26, 2011

I am assuming that you want to decrease the stroke at the pedal. Or stated another way, you want more master cylinder stroke for a given stroke of the pedal.

When you re-drill the hole for the pedal pushrod clevis 1" lower, you are changing the pedal ratio from 6:1 to 4:1. This means that the pedal travel will be 2/3 of your current travel, and the force required at the pedal is going to increase by 50% for a given braking effort.

Another way of accomplishing a similar thing would be to change the master cylinder bore size from 1" to 1-1/8". The change would not be quite so drastic. The increase in bore area would be 26.6%. That means the force at the pedal would increase by that 26.6%, and the travel would be 79% of your current combination.

So doing this would be the equivalent of losing boost. It might even be enough that you'd need less pedal force with no booster at all (though the stroke would be the same as now).

To keep from having to use the angled mount, you could rework your firewall and mounting bracket to mount the booster 1" lower on the firewall. This would be a lot of work, including making a new hole in the firewall for the back end of the booster. (That is how they did it on 60s cars.) What you also will find is that decreases booster to valve cover clearance, and that may be the best reason not to do it that way.

I have never seen a Z'd pushrod offered commercially, and probably there's a good reason for that.

I also think that trying to angle the pushrod is not a good idea even if there was no apparent bind. It would side load the input end of the booster and wear it prematurely.

The cast iron master cylinders that many of us use are meant to be mounted flat or close to it. If you have the 4 wheel disc brake Corvette cast iron master cylinder, there is lots of reservoir volume, and lowering the level slightly won't cause a problem.

Another solution for an angled booster is the later model aluminum/plastic master cylinders with the angled reservoirs.

thomaswatk · Oct 26, 2011

Why not just mount the booster flat against the firewall and keep factory angle? I have always been confused about the need to drill a hole 1" lower because of angle issues. I understand the ratio factor, but is it really necessary if the booster is mounted flat on the firewall ?

Rick_L · Oct 27, 2011

thomas, there are lots of issues here, and reasons things are done the way they are.

Once you get to a certain diameter of brake booster, you'll run out of clearance to the valve cover; angling the booster/master cylinder increases the clearance. It's pretty common on OEM isntallations for the same reason.

You can see from my above post that some installations (in fact most OEM installations) use a booster, in fact a bigger booster than is really needed, then the pedal ratio and master cylinder size are sized in such a way that a lot of that boost is negated by those. The result though is that the pedal travel is very small, it doesn't move much from the pedal being fully up to having the brakes fully applied. It's just the way cars are made, and they make them that way because people like them that way.

Old Buzzard · Oct 27, 2011

My $.02....
That 7" booster is going to be at the "lower end" of the performance scale. [Unless you have larger than usual rotors. [IE: 13" vs the usual 10.+" in most kits.]

I have the 8" dual, 1 1/16" MC, and the pedal undrilled. With SIGNIFICANT pedal psi, I have 900psi at the rotor. [1200psi is where I'd like to be.] [Vac is 15-17" @ idle]
FWIW: many brake kits are "rated" with the booster seeing 20".
I have switched to soft pads, [organic], changed to the large bore calipers, [2.75"], and have decent brakes. [NOTHING to "write home about"].
The 2nd class levers u r dealing with don't help, either.

Like I said, my $.02....

pcdarks · Oct 27, 2011

Recently having done a disc conversion one of the things I don't like is that the pedal goes nearly to the floor before the brakes function. Is this modification to bring the pedal higher? A few washers and re drilling the pedal seems pretty easy if it would fix this.

Rick_L · Oct 27, 2011

Yes but at the expense of having to push harder on the pedal to stop the car. You don't want that do you?

Do you have drums on the back? Do you have a residual valve in the line if so? Do you have the drums adjusted pretty tight? Both those things will bring the pedal up without other consequences.

pcdarks · Oct 27, 2011

Rick_L said:
Yes but at the expense of having to push harder on the pedal to stop the car. You don't want that do you?

Do you have drums on the back? Do you have a residual valve in the line if so? Do you have the drums adjusted pretty tight? Both those things will bring the pedal up without other consequences.

Rear drums,I assume there is a residual valve in the rear,drums are adjusted. I don't mind pushing a little harder as nothing can be a hard as stopping four drums without power assist which is where I came from. Also the hole could be drilled at 3/4" improving pedal height but not sacrificing leverage as much.

warren57 · Apr 3, 2012

new install

I just finished my install and the bracket supplied with the cpp mc and booster is angled. The angle sets the inline plane of the pushrod that was re-drilled 1" lower at the pedal. When dropping it one inch the rod pushes up at a slight angle as the pedal is pushed. The mount compensates for this angle. As for stroke length, I measured length of master cyl stroke by manually pushing it in not attached to the booster or linkage. With the linkage 1" lower it still pushes in the full length of the cyl stroke.
I am getting ready to bleed the lines and see how it all works, but it appears to be correct.

Brake pedal modification for increased travel.

jeepjoint

595544

Rick_L

thomaswatk

Rick_L

Old Buzzard

pcdarks

Rick_L

pcdarks

warren57

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