This is the brake pedal from my '67 Camaro. The hole closest to the top pivot hole (arrow
Pedal Travel Blues
Gary Tetu, Scottsdale, AZ: I love your magazine but wanted to let you know what happened after my professional shop installed a Wilwood 7/8-inch bore master cylinder in my '70 Plymouth Duster. My Duster has Wilwood disc brakes in front with drum brakes in the rear. I did not get the same impressive results you reported in the What's Your Problem section of your Mar. '10 issue. In the piece titled "Take a Brake," you reported a "very firm pedal" with "far less pedal effort necessary to generate the same braking force."
In my case, the brake pedal became soft and mushy and had 1.5 inches more pedal travel. The pedal effort required to generate the same braking force was only very, very slightly improved. My shop properly bled the brakes and tried multiple adjustments of the proportioning valve. It also tried a 10-pound residual pressure valve in the rear brake line to keep the rear brake shoes partially expanded and thus reduce brake pedal travel. To me the mushy brake pedal and increased pedal travel were just not worth it, so I had the shop reinstall my OEM 1-inch-bore master cylinder. Now I once again have a rock-hard brake pedal that requires little travel. So what if I have to push the pedal a tiny bit harder.
Just thought I would let you know my results. I wonder how other readers have or will make out with this modification. Based on my results, I hope they are not expecting this 7/8-inch bore master cylinder to feel like modern booster-assisted power brakes.
Jeff Smith: I should know by now that there are always exceptions, Gary. But that does not mean there isn't a way to allow you to have your cake and brake it, too. I've been spending a lot of time with my buddy Doug Norrdin at Global West Suspension, and in the middle of one of our many discussions on how to improve my '65 Chevelle's handling, I asked him to comment on your situation.
We actually ran into a similar situation on my '65 corner-turner Chevelle, moving from a 15/16-inch master to that same Wilwood 7/8-inch master cylinder you tried. When we had finished bleeding the brakes, I immediately noticed what felt like a soft brake pedal. We bled the brakes again but found no air in the system. What I was feeling was a change from the previous rock-hard brake to additional pedal travel (perhaps 1 inch-I didn't measure it) that was less than satisfying.
We talked about it and decided to test the car first before going back to the original master, which was showing signs of age. After posting the quickest overall time at the Run to the Coast Baer Brake Stop Challenge (see "Pro Touring Shootout," Aug. '10 CC), we decided that we could live with a little bit more pedal travel.
Doug's a pretty sharp guy, and he offered a couple of suggestions for your car. First, with all the air purged from the system, that mushy feeling is just the additional piston travel that is a direct result of reducing the area from a 1-inch to a 7/8-inch piston master cylinder. That's a huge change, but you don't have to live with that. Doug suggested looking under the dash to see if there may be two holes drilled in your brake pedal. On GM cars, the manufacturer used the same pedal for power brake- and manual brake-equipped cars, and there are two holes to attach the actuator rod. Doug says on average that power brake cars use a 5:1 pedal ratio while manual brake cars increase this ratio to 7:1. The closer the actuator hole is to the brake pedal pivot, the greater the pedal ratio. This ratio not only multiplies the force applied at the pedal but also affects the distance the pedal travels. The lower (power brake) hole not only decreases the force applied to the master cylinder but also increases the distance traveled.
I removed the power brake pedal from my '67 Camaro RS, measured the overall pedal length from the fulcrum to the center of the brake pad compared with the position of the two separate holes from the fulcrum, and then calculated the ratios. If the arm is 12 inches from the fulcrum to the center of the pedal and the uppermost hole is 2 inches from the fulcrum, this creates a 6:1 ratio. This is where the top hole is in my Camaro, but the bottom (power brake) hole measured 3.4 inches. That creates a much shorter 3.6:1 ratio.
Installing a manual 7/8-inch master cylinder on the car and using the lower (power brake) hole on the brake pedal would easily create a situation where the pedal travel is increased due both to the low ratio and to the smaller master cylinder piston diameter. Even with major leg force on the pedal, the pressure generated by the master cylinder would still be low because of the weak pedal ratio. To compute this difference, I used a 50-pound force applied to the pedal with a 6:1 ratio and calculated 499 psi of master cylinder line pressure. That same 50-pound force with a 3.6:1 ratio only generates 299 psi-a drastic 40 percent drop in pressure. In other words, you'd have to use 70 percent more pedal effort to create the same master cylinder hydraulic pressure.
It's possible that your Duster was fitted with power brakes from the factory and that the pedal ratio may be somewhere in the 5:1 range. If the pedal is not drilled for manual brakes, you can easily drill a 6:1 ratio hole. Change the pedal ratio and then bolt that Wilwood master cylinder back on the car. You should discover the pedal travel will be reduced and the hydraulic pressure will increase. You mentioned that the brake pressure measured at the calipers was only 800 psi, but I'll assume that was only with medium effort. With this smaller master and improved pedal ratio, the line pressure to the front brakes should increase to more like 1,100 to 1,200 psi. The pressure to the rear brakes will be less to prevent premature lockup.
Global West Suspension Components
San Bernardino, CA 909/890-0759