Jungle Drums Speak
David Bourhill, Roanoke, VA: I am rebuilding a corporate 10-bolt from a '77 TA with 3.42 gears to go under my '77 Formula. I need to replace the backing plates. Also, since drums are so cheap, I plan to replace them rather than turning them. Since I am doing all this, I thought, Why not up grade to bigger drums? I can have the drums redrilled to the Firebird pattern if necessary. What will fit? I was thinking of drums off a wagon. Will I need to change wheel cylinders and the proportioning valve? I prefer to use factory-type parts due to cost.
Jeff Smith: The Firebird drums used on a '77 TA are the standard 911/42-inch-diameter using 2-inch-wide rear brake shoes. We'll assume you are changing backing plates because the old ones are heavily scored where the shoes ride against them. For those car crafters who may not be able to find replacement backing plates, it is possible to weld these ridges in the plates, grind the positions flat, and polish them to offer a level surface for the shoes to ride against.
We looked up larger brake drum sizes, and the Parisienne wagon, for instance, uses larger, 11-inch rear drums but also employs a 5x5-inch bolt pattern. As you suggested, the drums could be redrilled, assuming the rear-axle mounting flange is the same, but this seems like a lot of work. We've seen references on different musclecar Web sites about converting the small 911/42x2-inch '64-'72 A-body brakes up to '73-'77 A-body brakes that are 11x2 inches. The axle bolt pattern for the backing plate should be the same, but you may need to open up the axletube hole slightly with a file to fit the later Firebird 10-bolt housing. We've not performed this swap, so we're going on faith here, but it sounds like it might work.
After a little more research, we discovered that Master Power sells exactly what you're looking for, David. The company offers an 11-inch drum-brake kit for '64-'81 Chevelles, '62-'74 Novas, and your-era Camaro/Firebird (PN DR1700K, $399) that will fit your '77 TA 10-bolt. The kit comes with all new shoes, springs, and wheel cylinders already mounted on brand-new backing plates.
Regardless of the size of the drum brakes or the lining material you choose, a complete flush and refill of brake fluid is an absolute requirement. Old brake fluid absorbs moisture, which reduces the fluid's boiling point. When the contaminated brake fluid heats up beyond the boiling point of water, the water turns to steam and air is introduced into the hydraulic circuit. This creates, at best, a spongy brake pedal and at worst, a brake pedal that sinks to the floor! This generally happens when you need the brakes the most. Most high-performance brake companies recommend replacing brake fluid at least once a year and more often if you abuse the brakes or run the car at track days on a road course. Do not use silicone (DOT 5) brake fluid because it has a nasty tendency to create air bubbles when the fluid gets hot. This fluid is good for cars that are mostly stored and only driven short distances, as it does not attract moisture, which will maintain the hydraulic circuit much better than the other DOT fluids. A good DOT 4 high-performance fluid is best for a performance street car. DOT 4 has a higher boiling point than DOT 3, which is why it is a better and more expensive fluid.
As you have surmised, David, even a minor change to the rear drum brakes will require testing and changes to the rear-brake proportioning valve. If your Firebird has a factory, nonadjustable combination valve, you really should exchange or bypass it for an aftermarket adjustable valve from Master Power, for example (PN VL3350W, $89.95). This valve allows you to fine-tune the amount of pressure reduction to the rear drums to prevent premature rear-brake lockup. This happens on a regular basis with modified cars where the owners do not carefully adjust these valves. Testing the rear brakes should be done in a controlled environment where there is no chance of hitting either a stationary object or another car. In other words: Don't test your junk on the city streets. Go to a location where you can safely accelerate and brake to adjust the pressure. Start with the pressure to the rear brakes low and gradually increase the pressure until the rear brakes lock up before the fronts. Then return to the best previous setting. Keep in mind that any changes to rear tire diameter or weight distribution will affect brake performance.