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  #1  
Old 04-05-2007, 08:41 AM
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Join Date: Apr 2007
Location: L.I., NY
Posts: 3
Brake pedal drops when parked overnight

Hi all, I'm a new member to this forum from L.I., NY. I drive a '94 E320 w/90K on the clock. I do most of the work on the car myself as a hobby. My present problem with the brakes is this; when parked overnight or extended period of time, the brake pedal drops to to floor, one pump and the pedal pressure is fine. On extended highway drives, the pedal sometimes also drop very low when I step on it by a toll booth or exit ramp, one pump the pedal pressure returns to normal. This been happening several months. Master cyl seems fine, it holds pressure while stepping on it hard with engine off and with engine running. Brake booster seems fine, normal pressure needed to stop car. I bled the brakes a few months ago, pedal pressure fine a few weeks, then reoccurring problem. I then changed the front rotors, pads, rear pads, flushed brake fluid; pedal fine for a few weeks, now it's dropping again. Anybody got comments on this? It's driving me nuts. Can't figure out where's the problem.
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  #2  
Old 04-05-2007, 09:41 AM
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Join Date: Dec 2004
Location: Nashua, NH
Posts: 3,956
Master Cylinder. If there are no external leaks I can't see how anything else could be at fault. I believe the condition you are describing is typical when the internal seals are begining to fail. When cold the seals shrink a little allowing the fluid to pass around them, after you pump it a couple of times and some heat is generated the seals swell back to close. It's dangerous to drive it in my opinion and should be replaced, TODAY.
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  #3  
Old 04-05-2007, 10:21 AM
david s poole
 
Join Date: Mar 2005
Location: dallas
Posts: 1,822
absolutely the master cylinder is starting to fail.
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"Fortune favors the prepared mind"
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2000 Mercedes Benz C280
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  #4  
Old 04-05-2007, 10:51 AM
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Join Date: Oct 2004
Location: Baton Rouge La
Posts: 2,621
i wouldn't drive it... this happened to me before in another car.... that said.. i did drive it to a dealer..and they switched it out in under 30 minutes.
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  #5  
Old 04-05-2007, 10:51 AM
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
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My VW was really bad about that sort of thing, and it was always the master cylinder. For some reason, either it was a bad design, or the extreme weather changes through the seasons would cause the seals to prematurely fail.
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  #6  
Old 04-05-2007, 12:55 PM
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Join Date: Feb 2003
Location: roslyn, LI,NY
Posts: 445
low brake pedal pressure

There could be several reasons for this. a)s mall leaks in master cylinder, b) small leaks disc brake cylinder seals, c) inadequate bleeding brake lines. Does fluid level change in master cylinder change? Hope this is possible answer. Abe G
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  #7  
Old 04-05-2007, 05:27 PM
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Join Date: Apr 2007
Location: L.I., NY
Posts: 3
Thanks guys, I'll r/r master cyl asap. There's no loss of fluid in system, no leaks from wheel cyl's, and good pedal pressure after one pump. I'll change the master cyl and report back any change in pedal drop. Anyone got a link to r/r'ing a master cyl?
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