hey dave , i'm having trouble holding my car back with the line lock applied. I have a 1980 malibu with the d154 two piston willwood calipers with a factory master cylinder. Do you think it is possibe to move my brake pedal pivot point up to achieve a better brake ratio. I think I have the hole in my brake pedal for a manual set up. Do you see any harm in doing this?
Tim,
Are you running a manual brake setup or a power boosted setup?
The Wilwood twin piston calipers have a piston area of 4.12 square inches. A stock 2.5" diameter piston caliper has a piston area of 4.9 square inches. Piston area is important because that translates into rotor torque or clamping force of the caliper. Usually, the larger the piston area the better, but it needs to be matched to the correct size master cylinder. You have three issues working against you.
1. smaller piston area calipers (less clamping force)
2. larger master cylinder piston bore (less line pressure)
3. pedal rato is low. (move effort by the driver's leg to stop the car)
The factory, power boosted master cylinder has a bore size of 24mm. From 1978 to 1981, these are normal, strait bore master cylinders. These master cylinders came in cast iron and aluminum. Since you have a 1980 Malibu, you may have an aluminum one. Larger bore master cylinder deliver more volume of brake fluid, but they will create less line pressure. Manual brake equipped "g-bodies" came from the factory with a 7/8" bore master cylinders. Smaller bore master cylinder create more line pressure (potentially more clamping force) with less volume. The best combination is a caliper with the largest piston area and the smallest master cylinder possible to supply to amount of fluid volume to fill the caliper. This creates the greatest amount of clamping force at the rotor.
From the factory, the master cylinder was moved up on the firewall and used a pin that was higher up on the pedal for manual brakes. Moving the pin up on the pedal will give a better pedal ratio. This means the driver will have more leverage to operate the brakes and the driver does not have to use more leg strength to stop the car.
Moving the master cylinder up on the firewall and raising the pivot point of the push rod deeps the push rod geometry in check. Moving the pin up on the pedal and moving the pivot point of the push rod up on the pedal WITHOUT moving the master cylinder up creates two things.
1. Side loading of the master cylinder piston
2. Incorrect geometry.
Side loading of the master cylinder piston will prematurely wear out the seals and the master will start leaking and loose pressure.
Incorrect geometry is a little harder to explain. Think of the brake pedal assembly as a swing set. As you swing on a swing, the seat you sit in on the swing goes through an arc. As you swing, you go up high, come back down toward the ground, and then go back high again. Your brake pedal pad and push rod pin do the same thing, but they start out in the high position. As the brake pedal is pressed, the pedal pad and push rod pin arc down. At half pedal stroke, your at the bottom of the arc and if you keep pressing the brake pedal fully, the pedal pad and push rod pin will start to arc back up.
On a g-body manual brake set up with a stock 6 to 1 pedal ratio means that if the brake pedal pad moves 6 inches, the push rod pin moves 1 inch. On the g-body pedal assembly, the pushrod start out being aligned with the master cylinder piston. As the brake pedal is pushed, the push rod will have a very little upward angle of the push rod at mid pedal stroke (3 inches of brake pedal pad movement, half inch of push rod pin movement). If you press the brake pedal down all the way (6 inches of brake pedal pad movement, 1 inch of push rod pin movement), the push rod will come back into alignment with the master cylinder piston. The brake pedal push rod pin has moved one inch, but also the master cylinder piston has moved one inch. This is how its suppose to work.
If you move the pin up on the pedal without moving the master cylinder up, the push rod will be at a severe downward angle. At the mid point of the pedal stroke (3" of brake pedal pad movement/) the pin is at the bottom of its arc and the push rod is still at a less severe angle. Past the mid point of the pedal stroke (3 to 6 inches of brake pedal pad movement) as the pin will start to move up on its arc, pulling the pushrod up and away from the master cylinder piston. . So past the mid point of the pedal stroke, the pushrod is NOT moving the master cylinder piston at the same ratio that the pedal pad is moving. Bottom line is, besides the side loading of the master cylinder piston, the master cylinder piston will not move the one inch needed to operate the brakes effectively. Past the half way point, the harder you press the pedal will not translate in moving the master cylinder much further on its piston stroke.
Clear as mud.
I hope I explained it pretty good. Let me know if you have additional questions.
