Brake Talk
Richard Rochelle, Chicago, IL: I've got a '69 Camaro with a 12-bolt rearend and a 350/350 combo. I've heard about putting Ford Explorer brakes on the rear of a Chevy rearend. This sounds like an economical way to go. I've got discs on the front from a Chevelle and an adjustable prop valve. Would this be the way to go or would I be better off with a kit? Saving money would be great and a junkyard crawl is always fun.
Jeff Smith: This is a great question, Richard, and one littered with potential land mines. There are plenty of people out there who are mixing and matching brakes parts on their cars successfully, but we'd hazard a guess that most of them (like us) really just got lucky with the combination. Here's what we've learned. We've seen firsthand what happens when adding a rear disc brake package that is not engineered to work with your front brake combination. We have a friend with a killer '69 Camaro who did a 60-to-0 braking test on his car only to discover that the rear brakes always locked up ahead of the fronts. He backed his adjustable proportioning valve off all the way and the rears still locked up prematurely, which put the car into a dangerous loss of control. What he eventually learned is that the rear Ford Explorer calipers use very large rear caliper pistons. Even minimizing the rear pressure (we'll get to that detail later) did not fix the problem, because the greater volume requirement of the large rear pistons demanded more master-cylinder piston movement. This led to the rear brakes locking up first because the pressure was still too high.
We spoke with the tech folks at Baer Brakes, and they were very familiar with this situation. Their safety margin is to recommend that the front calipers be a minimum of 25 percent and preferably 40 to 50 percent larger than the rear calipers. So let's say that your front Chevelle calipers use a single floating piston that measures 2.8 inches in diameter. Using Baer's recommended 50 percent figure to be safe, that means that the total rear-caliper piston should be no larger than around 2 inches in diameter. Keep in mind that this is really about piston area, using the formula Pi times radius squared, and the radius of a circle is half its diameter. So if the area for a 2.8-inch front piston is 6.15 square inches (1.4 x 1.4 x 3.1417 = 6.15), the rear caliper must not exceed a piston area of around 3 square inches. This calculates out to roughly a single rear-piston diameter of 2 inches, or multiple pistons whose areas add up to 2 square inches. Front or rear calipers that use pistons on both sides of the rotor generate what is called opposing forces, so you only calculate the total piston area based on one side.
Also, the master cylinder must be able to supply sufficient volume to both the front and the rear brakes. Basically, a smaller master-cylinder piston generates more hydraulic pressure with the same pedal effort but does so at the cost of reduced fluid movement. This means we need both pressure and fluid volume, and sizing the master cylinder must also be part of the equation. In our experience with Chevelles and Camaros, a master-cylinder piston diameter of 71/48 inch to 151/416 inch is a good compromise between pressure and volume. This can also be used with manual brakes (no vacuum booster) with reasonable pedal effort.