I have front disc from a 69 Firebird on my 69 Camaro and rear disc from a 78-80 Cadi.
The piston dia. on the fronts are 2.935
The dia. of the rear piston is 2.495. Is this going to be about right for this vehicle?
Thanks.
Guy

Here is something posted a while back by dnult.

I was reading an old Camaro Performers Magazine the other day (Winter 2002) and discovered an article about braking. I found it interesting and wanted to share what it said. I'm currently fiddling with brakes on my 68 Camaro. This article at least put the problem into better prespective. As with all rules of thumb, these are to get you close. The optimal spec for your car may fall outside these specs. Here's what the article said...

1) The angle between the push-rod and pedal should never be less than 90* or a soft pedal under hard braking will result. This will be more of a problem on small bore master cylinders that stroke a fair amount.

2) The Caliper to Master Cylinder piston surface area ratio should be between 12% and 19%. (Not sure if this is true for both Manual and Boosted brakes). Take the surface area of the MS piston divided by the suface are of one front caliper (d^2 / 4 * 3.14159). If using four piston calipers, take the sum of one side for caliper area. In other words sum the surface area of two pistons. The opposing pistons will apply similar force once the pads contact the rotor and cancel out. My 1" bore MS and 2.84" single piston caliper result in a ratio of 12.4% It was interesting to note that a 1.125" MS resulted in a caliper ratio of 15.7% which is right in the ball-park, but I believe experienced users will say this is a firm pedal.

3) The pedal ratio should be between 4:1 and 6:1. To calculate, take the distance between pedal fulcrum and pushrod attachment point. Divide that lenght by the distance from fulcrum to peddle centerline. The article further sited a recommendation of 4:1 for power brakes and 5:1 for manual. My 68 is 2"/12.5" for a 6:1 pedal ratio in the manual brake setting. The boosted position is another 1 and 1/2" lower resulting in a 3.5:1 ratio (low by comprison to 4:1).

4) The suface areas of the front caliper piston (for 4 w disc) should be at least 25% larger than the rear piston surface area. They go so far as to recommend 40% larger and site a 93-97 camaro as being 55% larger. What they don't say is if there is a relationship between this ratio and the rotor size. In other words, if the rotor size is not 1:1, how does this rule of thumb change?

While I was on a kick with brake math, I calculated the pressure provided by a vacuum booster. Using a 9" booster at 17"Hg, I came up with 531 lbs of force applied by the booster. Now I am looking for details on how the booster works. Surely this pressure is reserved until the brakes are applied. 531 lbs is enough pressure to create 417 lbs of line pressure with a 1" MS. That's more than enough to overcome the pressure of the spring in the MS. Still trying to noodle that one.

Hopefully this is useful information. I now feel a bit better about my 1" bore MS in a manual application. The resevoir size is my only concern at this point. But the quest continues.

-dnult gjohnson,
What I'd consider a good example of front to rear proportioning is the 69 JL8 system which uses four piston calipers front and rear and 11.75" rotors.

Here are the dimensions, remember we need to compare bore in square inches, not just bore dia.
Another rule is, only compare one side of each caliper, so compare the bore area of a front single piston caliper to the area of a single piston of the rear caliper. If you compare a single piston to four piston caliper, use just one side of the four piston caliper, (one side). If the caliper has six pistons, add the sq in of each piston from one side of the caliper.

one ft cal piston 1 7/8" (1.875) 2.7611654186 sq in
two ft pistons: 1 7/8" each 5.5223308371 sq in 64%

One rear piston 1 3/8" (1.375) 1.4848934029 sq in
two rear pistons 1 3/8" (1.375) 2.969786806 sq in 36%

f/r total 8.492 sq in
-------------------------------------
Piston sq inch formula:
.7853981635 X bore, then squared = sq in of bore.

For front percent, add F/R sq in totals, divide front piston area by total area, = front percent of total piston area.

Example,
5.5223398371(F), + 2.969786806(R) = 8.193095177 total.
5.5223398371 / 8.193095177 =.6375, multiply that by 100 to get percent. 63.75%

On a Camaro, I'd like to see just a little over the 64% front braking designed in, then use an adjustable proportioning valve to fine tune it. You can turn the rear down but you can't turn it up if the rear pistons are too small.
David

[ 04-19-2004, 01:02 PM: Message edited by: davidpozzi ]

"I have front disc from a 69 Firebird on my 69 Camaro and rear disc from a 78-80 Cadi.
The piston dia. on the fronts are 2.935 6.4918066952 sq in

The dia. of the rear piston is 2.495. 1.959568418 sq in."

total sq in: 8.451375113 = 76% front brakes.
While this will work, it's not going to ever lock a rear wheel! You won't have optimum rear percentage.

On the firebird and camaro those rear calipers are used with the GM "metric" front calipers, I may be wrong, but I think those have a bore of 2.5" sq in of:1.963495409, this yields a F/R percent of 50/50. which would require a proportioning valve to reduce the rear pressure to get a more ideal 36% rear bias.
One dis-advantage of too large a rear bore is it will take more pedal effort to stop the car because you lose ft clamping pressure by reducing ft bore area, and the rear can't be used full-on to make up for it.
Too large a rear piston makes you lose pedal travel.
Hope this helps.
David