1968 Chevelle Brake Booster - Booster Blues

Troubleshooting Brake Booster Befuddlements

Randy Fish Feb 1, 2004 0 Comment(s)
Sucp_0402_01_z 1968_chevelle_brake_booster Master_cylinder 1/7

Thanks to modern technology, trick brake systems are available for most any budget. The days of choosing Original Equipment and nothing but Original Equipment are a thing of the past. However, not every car guy on the planet is in the habit of servicing brake systems for a living. Therefore, not all of us can spit out the normal troubleshooting regimen-in order-from the first piece on the checklist to the last. And, like driving to an unknown destination, there's always a right way (and a wrong way) to get there.

If you read "Wagon's Whoa" recently (SC, November 2003), you're aware of our brake upgrade on a donor '68 Chevelle wagon using a kit from Master Power Brakes. This car's previous owner had told our buddy that lots of new parts were recently installed before the "For Sale" sign went on it-including a front disc upgrade and a new power brake booster. In reality, the guy used a standard replacement, drum-brake booster, along with an upgraded 11-inch disc-brake package. Then, we upgraded it once again, using 12-inch rotors. The swap went smooth as silk and we made our editorial deadline with no problems. However, a lengthy road test proved we had a funky pedal. Obviously, we had some components that just weren't talking to each other.

Back to square one, we double-checked the bleeding process, only to find no change in the condition. Then, our thoughts turned to the proportioning valve. Could that be the problem? Well, like we said, not being brake service technicians, we were just guessing. Deciding not to run out and buy a new proportioning valve (about a $100 pop) just because that "might fix it," we called Master Power Brakes for some technical help instead.

Sucp_0402_03_z 1968_chevelle_brake_booster Metering_valve 2/7

Metering ValvePresent in the front disc portion of a disc/drum set-up, the metering valve allows the slower reacting rear drums to contribute to the overall braking performance. Actually, it slightly retards the application of the front discs, which in turn, allows the rear drums to catch up. Without a metering valve, your car would nose dive when the brakes were applied, and therefore, you'd see excessive front pad wear. Not good.

When explaining our situation, they dialed us in to the proper trouble-shooting regimen, and everything began to make sense. Following is some general info on various brake system valves and the jobs they accomplish.

Common Brake System Valves
In order for any brake system to operate properly, some type of valve is needed, which will allow your vehicle to receive the correct front-to-rear brake balance. Four different types of valves are common in today's systems. Let's take a look.

Now, getting back to the problem at hand-hard pedal. In talking to Master Power's tech rep, we learned that a faulty power brake booster could cause a very hard pedal, which in turn causes scary stopping. Scratching our heads, we figured, "How can the booster go bad just by changing the front brake system?" Well, it didn't. Next, we learned that power brake boosters need 18-inches of vacuum (from the engine) in order to operate properly.

Anything less than 18-inches can also cause a hard pedal. However, on high-mileage cars, or high-horsepower engines with lumpy camshafts, you'll see less than the prescribed 18-inches of vacuum. For cars with aggressive camshafts (or those producing less vacuum), Master Power recommends using an accessory electric vacuum pump.

Sucp_0402_04_z 1968_chevelle_brake_booster Combination_valve 3/7

Combination ValveMost common in today's applications, a combination valve incorporates both metering and proportioning. Combination valves are readily available for disc/drum, as well as four-wheel disc systems.

With most mechanical malfunctions, a chain of events usually contributes to the end result (or problem). The guys at Master Power explained that before the car was purchased, swapping from a factory power drum set-up to a factory-style 11-inch, power disc system was where our dilemma actually began. Now, when we went to an even larger-diameter rotor (though only by one more inch), it needed more booster volume for proper application of the front brakes.

Therefore, we were advised to run Master Power's dual-diaphragm power booster, which produces a greater volume of vacuum, as opposed to a standard, stock-replacement booster. This type of booster features a smaller canister, offering more valve cover clearance, but it also produces a greater volume of vacuum-exactly what we needed with this twice-upgraded brake system. Once you see the system explanations and problem-solving diagrams in Master Power's catalog, everything seems to make perfect sense. And, like driving to an unknown destination, there's always a right way (and a wrong way) to get there.

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