Summit Racing Equipment invited us on their podcast - OnAllCylinders - to talk all about brake systems. LEED Brakes' Vice President, Bill Cummings, and R&D Manager Dan DeKruger spent an hour answering vital brake questions pulled right from the LEED and Summit Racing tech lines. You can catch the entire interview below, or read 10 snippets from the chat that can give you an idea of the range of brake topics covered. If you’re mulling a disc brake conversion or other brake upgrade for your classic or vintage car, or are just struggling with a squishy pedal, consider this episode of OnAllCylinders a must-listen. 



10 Questions on Brake Tech for Classic Cars & Trucks with Leed Brakes


1. What’s the Difference Between a Brake Proportioning Valve, Combination Valve & Distribution Block?

“There’s kind of an assumption that anytime you see a brass block with a bunch of lines coming out of it, that it’s a proportioning valve. But depending on the year of the car, a lot of times, it’s not—if it’s a four-wheel drum car, it’s probably just a distribution block.

“A distribution block is really just a manifold, splitting lines from two-in to four-out. It doesn’t do any actual proportioning. Later cars that have a front disc/rear drum, those actually have a combination valve that includes proportioning.”

2. Why Do You Need a Brake Proportioning Valve?

When you have a dissimilar braking system—discs in the front, drums in the rear—that’s when you need that proportioning valve. The rear drums will lock up if they receive as much pressure as the front disc brakes, just because of they way that they function.

“That’s where it’s important to have that pressure reduction to the back of the car.

“A four-wheel drum car—or even a four-wheel disc car—a lot of times you don’t need that because you’ve got similar brakes front and rear. So if the brakes are sized correctly, and if the tire/wheel combination is sized correctly, the car can run equal pressure front and back. You won’t get that severe lockup in the rear brakes.

“That’s where it’s important to understand what you’ve got, what you’re converting to, and what you’re going to need as an upgrade to what’s on the car now.”

3. Can a DIY’er Adjust Their Brake Proportioning Valve at Home?

“It’s something you can do in your driveway. Start with the valve in the middle, then drive the vehicle for 200 miles to seat-in the pads, just normal driving. After that 200 miles, in a safe area, do some heavier stops to see how the rear brakes react.

“If the rear brakes are locking up, you can reduce that pressure a little bit. Or if you feel you can get a little bit more from the back brakes, you can increase that pressure.

“Where you set it really depends on the vehicle. Your tires, your suspension setup, the weight of the vehicle—there are too many variables for us to specify a particular setting.”

4. How Do You Begin Troubleshooting a Squishy Brake Pedal?

“The first thing we ask a customer is if they’ve bench bled the master cylinder. The master cylinder has to come off to be bench bled—you cannot bench bleed it on the vehicle.

“You don’t necessarily bottom the stroke out on the master cylinder by pressing on the brake pedal. So if you were to leave it on the vehicle, you might not get that last quarter-inch of travel out of the master cylinder—and that’s where the air will hide.

“If you take it off the car, you can use a screwdriver to physically push the piston in and bottom it out. That’s why bench bleeding a master cylinder off the vehicle is important—it’s [laughing] definitely not us just giving you extra work to do.”

5. Explain the Role of Bore Size in a Master Cylinder.

“Generally, the larger the brake calipers, the larger the bore you’re going to need. But what’s confusing is, just because you have a four-piston or two-piston caliper or one-piston caliper, doesn’t automatically determine your bore size—it’s determined by the piston size inside that caliper.

“It’s the total hydraulic surface area of all those caliper pistons that determines how big a bore size you’re going to need.”

6. Why Do Manual & Power Assist Master Cylinders Have Different Bore Sizes?

“In some cases, it depends on the manufacturer—everybody does things a little different. When GM went from manual to power brakes, it changed the brake pedal ratio too. That’s why you’ll see, like on a Camaro or Chevelle, the booster points up at an angle. That’s to hook up at a lower point on the brake pedal.

“So because of that change in ratio and all the assist that’s being added with the booster, if you were to stick to the same bore size for manual brakes, the pedal would be really spongy. By going to a bigger bore size, it maintains that pedal feel, but you still have the power assist.”

7. How Do You Fix a Soft Pedal That Goes to the Floor When Vacuum Kicks In?

“We get that question a lot—it’s absolutely not a bad booster.

“A customer will install the system, bleed the brakes, and pump the pedal up—all without the engine running. They’ll think they’ve got a good strong pedal, but once they start the car and the vacuum is applied to the booster, now that pedal is extremely spongy.

“It’s not that the booster did anything wrong—it’s that there was something wrong before they started the car. They didn’t have all the air out, they had mismatched parts, whatever the case may be, but they think that, since the pedal went to the floor once they started the car, that the booster must be bad.

“In reality, that soft pedal is a sign that the booster is doing exactly what it should, and that there’s something else in the system that we need to look at.”

8. Can Power Brakes Work With Low Vacuum Created by Aggressive Cam Overlap?

“Essentially any booster, regardless of diameter or master cylinder bore size, is going to need at least 15 inches of engine vacuum, minimum, for the booster to really function correctly. So as soon as you put that high-overlap cam in there, the vacuum is gone and your booster is never going to operate efficiently.

“There are some options. If you’re close, sometimes a change in bore size can get you back to where you need to be. But the solution we give to most of our customers in that situation is an electric vacuum pump.

“That’s going to give you 18 to 22 inches of vacuum, all the time.”

9. Is Engine Vacuum Affected by Elevation?

“Absolutely. I don’t have the chart in front of me, but every 500 feet you go up has a pretty dramatic effect on your manifold vacuum.

“If you’ve got a fairly stock engine that made 18 or 20 inches at sea level, you may not really notice it at higher elevations. But again, if you’re just on the borderline at sea level, you’ll notice a dramatic difference at high elevation—and your brakes will get harder.

“We’ve seen solutions at high elevations where people have changed brake pedal ratio and put smaller bore size master cylinders on to cover up those problems. And even an electric air pump—but it’s less efficient at elevation.

“If you get much above 4,000 feet, it starts to become really complicated.”

How Vacuum Changes with Elevation

Vacuum (In.-Hg) Elevation Above Sea Level Approximate Example
29.92 0 Miami, FL
28.86 1,000 ft. Kansas City, MO
27.82 2,000 ft. Tuscon, AZ
26.87 3,000 ft. Billings, MT
25.84 4,000 ft. Moab, UT
24.90 5,000 ft. Denver, CO


10. What’s the Most Commonly Overlooked Aspect of a Disc Brake Conversion?

“Wheel size. There are solutions for 14 inch wheels and we offer quite a few kits that’ll fit—but rear disc swaps are a challenge with small wheels. We typically start at 15 inch wheels for rear disc stuff.

“Definitely the size and style of the wheel, and how that will work with a disc brake conversion, is something a lot of customers need to consider.

“It starts with size, but it really comes down to how that wheel is shaped and styled. Some of the factory wheels, regardless of size, just were never designed to clear a disc brake caliper. And vice versa—there are some wheels out there that are pretty small, yet will clear a disc brake caliper.”

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