Cammed Up and No BrakesChuck McGeorge, Livonia, MI: I have a '68 Chevelle with a 427ci, 435hp big-block that had great brakes until a couple of years ago when I decided to put in a larger cam. The specs for the cam are 0.575 inch lift with 305 degrees of duration at 0.006 inch tappet lift. The part number is a Crane 11-214-4. I also had the heads redone with 125 pounds of seat pressure at 1.88 inches of installed height with 315 pounds at 1.280 inches open spring pressure. I also changed over to roller rockers. The rest of the package consists of a TH350 with a 3,800-rpm Cohan stall speed converter and a 12-bolt with 4.11 gears.
While I was doing all this, I also changed my brakes to disc up front and left the drums on the rear. I used a smaller, dual-chamber vacuum booster that came with the brake upgrade and also a dual-reservoir master cylinder.
The car hasn't stopped very well since I did all this. With such little vacuum (7 inches of vacuum at idle) coming from my cammed-up motor, I put on a 12-volt vacuum pump that gives me 16 inches, which still isn't enough.
People have suggested putting an adjustable proportioning valve on it, but why would I restrict the flow to the rear when there isn't enough flow now? I have heard of a hydraulic-assist pump that works off my power steering, but I haven't looked into that yet.
Oh, the proportioning valve is located on top next to the master cylinder. I had to remove the line lock because it would not hold the front brakes for burnouts.
I would like to drive the car more, but since it doesn't stop very well, I am afraid to take chances.
Jeff Smith: We're going to have to make a couple of assumptions to answer your question, Chuck. First, we're going to assume that when you say your Chevelle doesn't stop very well, that you have to apply a lot of pedal pressure and the car still requires too much distance to stop. You mention that you installed the disc-brake conversion at the same time you bolted in the larger cam. It also sounds like the brake kit was not optimized, even if you had not chosen the larger cam, and the lower-idle vacuum certainly didn't help. The good news is that the fix is easy and not that expensive.
You're right that changing the proportioning valve will not help your problem. The proportioning valve only reduces hydraulic line pressure to the rear drum brakes when used in conjunction with front disc brakes. Drums require less line pressure than disc brakes, and even rear discs on a four-wheel, disc-brake-equipped vehicle require less line pressure in most cases because anytime the brakes are applied, dynamic weight transfer occurs, which shifts weight off the rear tires and onto the front tires. This means that less pressure to the rear brakes will prevent the rear tires from locking up under hard braking. This leads us to what may be your problem.
It sounds like the kit you purchased is using a master cylinder with too large a piston diameter. In any hydraulic system, there is a direct relationship between master-cylinder piston diameter and the size of the piston on the caliper. The smaller the master-cylinder piston, the more pressure it will make. Increasing the master-cylinder piston diameter will reduce the amount of pressure given the same amount of pedal effort. What the larger piston delivers is more fluid, but at a lower pressure. Add to this your reduced-manifold vacuum, which reduces the effect of the vacuum-assist booster. You said a vacuum pump didn't help much, which also reinforces the concept that the master-cylinder piston diameter is too large.
I'm going to go out on a limb here and guess the master-cylinder diameter is somewhere in the area of a 111/416-inch-diameter piston. What I've found works well, even in manual four-wheel disc-brake applications, is a master cylinder with a 151/416-inch-diameter piston. The tradeoff with this smaller piston is that it will require more brake-pedal movement to displace the same amount of fluid as the larger piston.