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Re: '91 9000 with "bad brakes" - diagnostics? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 22 Jul 2008 07:52:54
In Reply to: '91 9000 with "bad brakes" - diagnostics?, buckmath, Sun, 20 Jul 2008 12:27:27 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Brake boost is provided by the ABS system, so I'd make sure the ABS pump and associated stuff works first.
Ideally, see the car when it hasn't been started for a few hours. Barring that, get into the car, and with the engine off, pump the brakes 20 times or so. This should de-pressurize the system. Open the hood and check the brake fluid level. It should be ABOVE the full line - pretty close to the top of the reservoir. Turn the key to ON but don't start it. You should see the ABS and Brake Fluid lights, if even only for a second or two, and hear the pump start to run. Start the car, get out, and look at the brake fluid reservoir. The level should be dropping. Ideally, it'll drop to the FULL line and the pump will shut off. The ABS and Brake Fluid lights should have gone out long before.
If after you did the original brake pumping the fluid level didn't change, I'd suspect the accumulator. If the level was really low, and pumping brought it up to FULL, then the owner has too little fluid and probably has air in the system. Now, having air in the system doesn't mean something else very expensive isn't wrong.
Basically, if you can't get the fluid level to change, either by pumping or by starting the car, it could be the Accumulator or the ABS pump. If you hear the ABS pump run, then it is likely the accumulator.
Do the Stomp test to check the accumulator. But listen for the ABS pump to run.
If the accumulator is marginal (5-8 pumps to get the ABS and Brake Fluid lights on), it might be a bad MC. The MC could be porting too much fluid, basically 'wasting' it on boost.
Could it just be air in the system? Yes. But the other issues can be very, very expensive. A new ABS pump is $1000. You can get an accumulator cheaper. A rebuilt MC isn't too dear, but a lot of labor. And if there is air in the system, why? If the owner ran it out of fluid, that would suck in air, but why did it lose that much fluid? If he/she/it replaced a caliper and just 'forgot' to bleed it, that could be the issue, but again, suspect.
Given the cost of the brake components, assume the worst and work your way towards the least expensive issues.
posted by 192.249....
, Tue, 22 Jul 2008 07:52:54 <-- Viewing This Message
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