Front brake caliper pressure

[Argess]

It depends on how hard you press down, but for 128 lbs of pedal force, 64 lbs presses the 7/8" dia. front MC to achieve approx. 479 psi with a 4.5:1 pedal ratio... if that is indeed the ratio.

I too have never been satisfied with my brakes. I've made some relatively inexpensive changes that have helped a lot.

First I installed Hawk HPS pads in the front.

Then I found the balance bar was out of adjustment and was locking up due to a severe angle between the bar and the Heim joints. The bar should be perpendicular to the cyl rods WHEN the brakes are applied.

Then I changed the front calipers. Turns out GM made their D52 calipers with quite a variety of piston diameters. For example, the ones used on a 1974 Malibu are 2,15/16" in diameter compared to my original 2,3/4" pistons from a 75 Camaro. This applies more force to the pads with the same line pressure. A fairly cheap and easy mod.

Lastly I installed EBC Yellowstuff pads in the Jag rear calipers. Hawk doesn't make pads for the Jag.

So I decided to calculate the foot pedal force for threshold breaking. For the first assumption, I’m going to assume that 0.9 Gs is around where the tires would lock up. The manufacturer of my car quotes a test value of 135 ft for 60-0 braking. This works out to 0.896 Gs so that helps support the 0.9 assumption. Also, they have achieved 0.9 Gs on the skid-pad, so although lateral skidding maybe not be the same as longitudinal skidding, they are probably close and once again the 0.9 G figure is supported.

So I went through some long (although not very complicated) calculations to calculate the pedal force required to get 0.9 Gs of braking. (I’m not going to bother showing these calculations here unless asked. Don’t want everyone falling asleep….)

With my original set-up, it takes 128 lbs of force on the pedal to achieve 0.9 Gs.

Once upgraded to the larger front calliper, it will take 117 lbs of force.

Although I found nothing so far for the 427 version, I did find two different specs on brake pedal pressure vs. stopping rate for the early 289 Cobras. One states 120 lbs for 0.93 Gs and the other yields 116 lbs for 0.93 Gs, the latter being an interpolated value from a table presented in the article.

By the way, I wanted to see how much 125 lbs of force felt like, so I braced myself against the bathtub and used my left foot to press a bathroom scale against the wall. It took a lot more effort than I thought and I have quite strong legs.

Finally I must admit that operating manual brakes takes some getting used to. You press quite hard and don't think it's braking well BECAUSE you are used to servo-assisted braking in your regular car.

All in all I'm satisfied with my brakes now, although I don't think I can lock the tires, it feels like it's on the threshold, which is good.