Brake booster problems can be both frustrating and dangerous, especially when the problem isn't with the brake booster itself. Because the Ford Ranger uses a vacuum brake booster, failure usually has to do with some sort of pressure leak, but it can also be due to a mechanical failure in the booster itself.
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Engine Vacuum Leak
By far the most common cause of brake booster failure is a lack of vacuum pressure. This is usually caused by a loose or cracked hose, which allows air to enter the system. Fortunately, this condition is usually fairly easy to recognise because vacuum leaks will cause often cause the engine to idle badly and hesitate on acceleration.
The vacuum brake booster has a large internal diaphragm which can harden and develop internal leaks over time. This condition is especially prevalent in cold, dry areas, which tend to hasten degradation of the diaphragm material. It's almost impossible to find a brake booster diaphragm sold as an individual unit, and it would be very difficult to replace if you could find one.
This is the second most common condition, and it can be diagnosed by checking for appropriate vacuum pressure and leaks in the hose. If there are no leaks in the hose, and the atmospheric port (air inlet) on the back of the booster is constantly drawing air in, then the diaphragm is probably bad.
An improperly adjusted brake pushrod will often cause either excessive pedal travel without complete engagement or short pedal travel and extreme operational sensitivity. Push-rods don't usually go out of adjustment on their own; this condition is often caused either by an improper replacement of an old master cylinder or loose retaining bolts. Once the original condition has been remedied, the pushrod should be adjusted so that it extends exactly .996 inches from the back of the booster (AutoZone).
The brake booster has a number of external ports that allow air to enter and leave it. The atmospheric (back) side of the booster has an air filter that can become clogged over time, which will gradually cause a reduction in power assist.
Similar in function to and working along side the ports, the springs that cause a vacuum booster's check valves to close can wear out or break over time. The check valves may become lodged open or closed, which will cause a power break failure. Failure of the vacuum-side check valve is often accompanied by an audible whistling sound.
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