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9000 Bulletin Board
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Vacuum and lights
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 24 May 2002 08:42:03 Share Post by Email
In Reply to: Brake servo losing interest overnight, Bill Davies, Thu, 23 May 2002 19:21:44
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First off, the 9000 does not use a vacuum system for power assist. The only exception is the non-ABS models. You didn't mention the year of your car or your location, but since the great majority of 9000's were ABS-equipped, I'm going to assume here.

When you drive off first thing in the morning, are the ABS and Brake Fluid lights ON?

The ABS-equipped 9000 uses the ABS system pressure to provide Brake Boost. The ABS pressure comes from a hydraulic (brake fluid) pressure accumulator - the 'bomb' - the metal sphere sitting on top of the brake master cylinder.

When the pressure in the accumulator drops enough, due to ABS activation or just hitting the brake pedal (brake boost), the ABS pump runs and pumps the pressure back up.

When the car is off, the pressure in the accumulator leaks down - the fluid flows back into the Brake Fluid reservoir. That's why the FULL line on the reservoir is well BELOW the top of the reservoir. That full line is when the accumulator is fully charged.

If the ABS pressure is VERY low, both the ABS and Brake Fluid lights come on together. This is a warning to show you have low pressure, and may not have brake boost.

If the car has been sitting a long time, overnight for an example, the ABS pressure has a long time to leak down. That's why when you turn the car on, you get the ABS and Brake Fluid lights both on. They usually wink off after a few seconds, as the pressure builds up over the minimum. This time may be a few seconds, or it may be 45 seconds to a minute. Longer times are associated with older cars, old brake fluid in need of a flush, a leaky valve at the base of the accumulator, low startup voltages, or just a lazy ABS pump.

If the ABS and Brake Fluid lights are both ON, you may not have sufficient pressure built up yet to have brake boost. This is especially true if you just drive off after starting, and use the brakes a lot - not only hasn't the pressure built up yet, every time you use the brakes, you take away pressure.

When you first start a cold car, you should see BOTH the ABS and Brake Fluid lights, even for a second or two. This tells you the lights are working. You should wait for those lights to go out before driving, because you know you have the minimum ABS pressure for brake boost at that time.

Of course, if your lights are going out quickly and you still don't have brake boost, I'm going to assume your brake master cylinder is on the fritz.

Good luck!

posted by 140.157.4...
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