Brake pedal goes down while sitting at a light.

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  #1  
Old 04-05-11, 01:45 AM
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Brake pedal goes down while sitting at a light.

1995 Buick LeSabre . low mileage: 30,500 miles, super clean. Problem: while sitting at a traffic signal perfectly still the brake pedal slowly goes down. One pump and it comes back and holds. No apparent fluid leaks anywhere? I haven't had this car long and don't know if it has always done it. Brakes work good with no pull left or right. I'm a tinkerer but by no means a mechanic. Any ideas? Could this be a vacuum leak somewhere? Any suggestions appreciated. Thanks in advance for them.
 
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  #2  
Old 04-05-11, 04:36 AM
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It may very well be that the master cylinder is not holding pressure due to worn or damaged seals which is not something you want to ignore. You at least need to eliminate this as the cause first. Next time you stop try putting full pressure on the pedal with both feet after you have pumped it up once and see if there is any additional pedal movement downward over a minute or so. Is your brake fluid topped up, have you had to add any lately, and do you know if the fluid has ever been changed by the previous owner. Any sign of leakage under the dash where the push rod comes through the carpet? Regardless of the cause of this issue, brake fluid needs to be changed about every 3 years as part of proper maintenance. With that low mileage the vehicle may not have seen many maintenance trips beyond ocaisional Thoil and filter changes, and if so there is a some possibility the old original OEM fluid is still in it, and now starting to damage your brake components for one thing as moisture has been obsorbed along with other chemical changes to the fluid itself.
 
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Old 04-05-11, 02:47 PM
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Will get right on it

Thanks for the insight and suggestions. All good points you make here. I'll be checking all this out. Thanks for responding so quickly. It's appreciated.
 
  #4  
Old 04-05-11, 08:33 PM
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Need to pump up brakes? If the pedal is soft and you need to pump it and release it several times before it firms up, it's usually a sign of air bubbles in the brake lines. The real question is why there's air in the brake lines, whether there's a slow leak somewhere, a loose bleeder screw, a leaking seal on the master cylinder. If the problem creeps up on you very slowly and you never checked the master cylinder, it's possible that the brake fluid level has been falling slowly as the parts wear, and the lid seal isn't perfect. If it's not strictly a post-turning issues, skip down two paragraphs for bleeding.

or, your brake booster is leaking, and pedal slowly goes down. normally, this comes with a slight hissing noise of air bled out.
 
  #5  
Old 04-20-11, 08:08 PM
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Thumbs up Problem's been dealt with thanks to the helping hand.

My thanks to equinox and ukrbyk for the suggestions. After further inspection I found that the Buick had new brakes installed all the way around. Recently too since they were still at about 95% wear. No leaks whatsoever anywhere. The brake fluid sure was funky too. I know DOT3 doesn't stay clear for long once it's in the reservoir but this stuff was extremely black. I wouldn't doubt that it was mostly the original fluid from 1995. :NO NO NO: It was the master cylinder causing the problem. A simple fix on this particular car. Brakes are holding fine now. Thanks for pointing me in the right direction. It's appreciated.
 
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