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Good brakes are directly related to good brake performance. For brake cars, good brakes require vacuum. Non-power brake systems must develop 1200 to 1400 lbs. of pressure at the calipers or wheel cylinders to work. Low brake pressure can occur when the wrong master cylinder is installed, the wrong size brake lines are used, the brake booster is bad, or there is low vacuum supplied to the brake booster.
There are several ways to check to see where the issue lies. The first item to check is how much brake pressure is being delivered to the calipers or wheel cylinders. A simple gauge can be used to test brake pressure. The gauge is screwed into the brake bleeder hole on the caliper or wheel cylinder.
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Our '55 car has a fairly firm brake pedal, but no matter how hard we stand on the brakes, they will not lock up. First, we checked the brake pressure at each wheel. The front calipers had 800 lbs and the rear drums had 850 lbs. This low reading may be the result of the wrong master cylinder, a bad brake booster, or low vacuum to the booster. We'll check it out.
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The brake booster operates using vacuum from the engine at the intake manifold or the carburetor base. This setup will supply full intake vacuum to the booster. As a rule, most brake boosters require a minimum of 18 inches of vacuum to work. Anything less than 18 inches of vacuum will result in a hard brake pedal, the inability to lock the brakes, and low brake pressure at the wheels. In other words, the brakes will be ineffective.
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To test manifold vacuum, a vacuum gauge should be connected to the base of the carburetor or to a vacuum port on the intake manifold. At idle, the reading should be at least 18 inches. If a low reading is obtained, check for loose or missing vacuum hoses at all locations. Ignition timing can also affect manifold vacuum. Our car has a fairly high-lift camshaft with a lot of duration, so the engine only had 8 inches of vacuum at an idle! This is why we have low brake pressure at the calipers and wheel cylinders.
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Unless you want to change the camshaft or convert to non-power brakes, a simple way to go is to install a remote vacuum pump (PIN 20-212). Our vacuum pump will generate 21 inches of vacuum at all times, more than enough to operate the brake system.
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The vacuum pump can be mounted anywhere on the car. A clean, out-of-the-way place to mount it may be the trunk area. A 3/8" vacuum hose can be run to the rear of the car along the frame or under the carpet and sill area. Be sure to use a good quality hose so that the hose does not collapse or kink.
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We decided to install our vacuum pump on the trunk ledge right behind the rear seat. For wagons, if the stock spare tire well is not being used, this is a great place to hide the pump. Our car has 18" wheels with wide tires that will not fit in the spare tire well, so it has been turned into a storage compartment. Alternatively, you can install the pump in the engine compartment, hidden behind an inner fender.
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The vacuum pump (PIN 20-212) has a 1/8" inlet where the supplied inlet filter should be installed. Using thread sealer, install the filter on the inlet, only tightening by hand.
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There is a red and a black wire on the motor. The red wire connects to the low-pressure switch on the vacuum pump. The black wire should be connected to a good body ground.
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The vacuum pump may be mounted with three 1/4" studded insulators. This will keep the pump very quiet.
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Drill three 1/4" holes and mount the vacuum pump in place. Run the black wire to ground.
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We removed one of the spare tire well covers and fed the vacuum hose running from the booster up through the spare hole and connected it to the brass fitting on the vacuum pump.
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Connect a wire from the ignition switch to the other terminal on the pressure switch. This wire needs to be fused and only hot when the key is on.
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With the brake booster now receiving 21 inches of vacuum, the brake pressure at the front calipers is now 1400 lbs and at the rear wheel cylinders, it is 1300 lbs! This made a huge difference in the braking on our car and will on yours as well!