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Old 04-05-2007, 08:41 AM
Registered User
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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Old 04-05-2007, 09:41 AM
Registered User
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.
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz
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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.
David S Poole
European Performance
Dallas, TX

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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Old 04-05-2007, 10:51 AM
Registered User
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.
1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride)

1994 e320 Cabriolet 98k (finally)

1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it

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Old 04-05-2007, 10:51 AM
G-Benz's Avatar
Razorback Soccer Dad
Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
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.
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car
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Old 04-05-2007, 12:55 PM
Registered User
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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Old 04-05-2007, 05:27 PM
Registered User
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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