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Explaination Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 2 Jul 2002 12:58:31
In Reply to: 90 900T Brake problem........, S-Works, Tue, 2 Jul 2002 10:13:42 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Background-
Brake boost is provided by pressurized brake fluid. The pressure is stored in the accumulator, which is the spherical unit on top of the master cylinder. This same pressure is used for the ABS system, but doesn't come into play unless the ABS kicks in.
Every time you hit the brake pedal, a little of the pressure in the accumulator is used for brake boost. Since no valve is perfect, the accumulator also leaks pressure when it just sits. The fluid returns to the brake fluid reservoir (the plastic thing you add fluid to).
The accumulator is pressurized by the ABS pump. When the car sits overnight, the accumulator pressure leaks down. That's normal. If the pressure is below 104 bar, both the ABS and Brake Fluid lights are on. If the pressure is below 140 bar, the ABS pump runs, and doesn't shut off until the pressure hits 180 bar.
So, when the car has been sitting, the pressure is very low - say 40 bar. You start the car, and since 40 is less than 104, the ABS and Brake Fluid lights are on. Since 40 is less than 140, the ABS pump runs, pressurizing the accumulator.
But it takes some time to get the pressure up. The lights don't go off until you hit 104 bar, and the pump doesn't stop until you hit 180 bar. It can easily take 45 seconds for the lights to go off (104 bar) if the car has been sitting, and another minute or more to get to 180 bar.
So what? Well, with no ABS pressure, you don't get pedal boost. At the 'magic' 104 bar, you have about 5-8 full stops before you lose boost. But if you start your car and drive right off with the lights ON, that pressure may be much less. And if you're using the brakes, you're using up the pressure as the ABS pump is providing it.
Some simple rules - don't drive off until the lights go out. Period. You might have brake boost, you might not. Letting the car warm up for a few seconds before driving off lets the oil get to where it's needed, extending engine life.
OK - so how long to get the pressure up? With an older car, it's normal for it to take longer to get the pressure up. Typically 45 seconds or so if the car has been sitting overnight. That's normal. If it's taking much longer, there may be a problem.
Do this - start the car, and listen for the ABS pump hum. Don't drive off. The lights should go out after a bit, and the ABS pump shut off about 2 minutes or so after starting. Now, hit the brakes hard and repeatedly. Count how many times you hit the brakes before the lights come on. If it's less than 5, you have a problem with the accumulator. It should be 12-15 before the lights come on.
Slow pressurization times (longer than about 45 seconds) may be due to a lazy ABS pump (expensive), leaky accumulator (only slightly less expensive), or old brake fluid. Brake fluid should be replaced every 2-3 years. Most people don't. That old fluid traps water, air, and gunk, corroding very expensive brake parts. If you haven't had the brake fluid flushed in the past two years, have it flushed and the system bled. It may speed up the pressurization time.
Sorry for the long post, but there's a lot going on here.
posted by 140.157.4...
, Tue, 2 Jul 2002 12:58:31 <-- Viewing This Message, Wed, 3 Jul 2002 11:27:55
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