Question about 1999 Saab 9-5

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No power brakes, brakes work, but not powered?

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Keep in mind two systems about this:
- VACUUM BOOSTERS; most power brake systems use a vacuum booster to assist braking. Vacuum booster needs two things to do its job: a good vacuum supply from the engine, and a good diaphragm. A vacuum supply hose that's loose, leaky, collapsed or restricted may not allow the booster to receive enough vacuum to provide the usual amount of power assist. Consequently, the driver will have to push on the brake pedal harder to get the same braking as before.

A restricted vacuum hose will cause boost to drop off when the brakes are applied in rapid succession. This happens because the blockage slows the return of vacuum in the booster.

To check engine vacuum, connect a vacuum gauge to the supply hose that runs from the intake manifold to the booster. A low reading (below 16 inches) may indicate a hose leak or obstruction, a blockage in the exhaust system (plugged catalytic converter, crushed pipe, bad muffler, etc.), or a problem in the engine itself (manifold vacuum leak, bad valve, head gasket, etc.).

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.


Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.



- HYDRO-BOOST; the system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide power assist.

Inside the Hydro-Boost unit, which fits between the master cylinder and brake pedal the same as a vacuum booster, is a spool valve and piston assembly. When the driver steps on the brake pedal, the pushrod slides forward and changes the position of the spool valve. This opens a valve port that routes power steering fluid into the cavity behind the piston to push it forward and apply the brakes.

Another component in the system is a pressure "accumulator." Some are nitrogen pressurized while others are spring loaded depending on the application. The accumulator's job is to store pressure as an emergency backup in case pressure is lost (engine stalls or power steering pump drive belt breaks). There's usually enough reserve pressure in the accumulator for 1 to 3 power assisted stops.

Problems with this system can be caused by spool valve or piston wear inside the Hydro-Boost unit, fluid leaks or loss of pressure (worn pump, slipping pump belt, etc.).

A simple way to test the Hydro-Boost system is to pump the brakes five or six times with the engine off to discharge the accumulator. Then press down hard on the pedal (about 40 lbs. of force) and start the engine. Like a vacuum booster, you should feel the pedal fall slightly when the engine starts, then rise.

The leakdown of the accumulator can be checked by pumping the brakes several times while the engine is running, then shutting it off. Let the car sit for about an hour, then try the brakes without starting the engine. You should get 2 or 3 soft brake applications before it takes more effort to push the pedal.

Slow brake pedal return may be caused by excessive seal friction in the booster, faulty spool action or a restriction in the return line to the pump. Grabby brakes are probably the result of contamination in the system or a broken spool return spring. If the brakes tend to goon by themselves, you've probably got as case of restricted return flow or a defunct dump valve. Excessive pedal effort can usually be attributed to internal leakage or the seeping of fluid past the accumulator/booster seal.

If a problem turns out to be in the booster itself, you'll have to replace it.



Hope this helps.

Posted on Sep 14, 2010

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