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Old 08-07-2002, 01:51 PM
Registered User
Join Date: Apr 2000
Location: Minnesota
Posts: 92
Help Brake question....Air or Master cylinder?

I am having this spongy brake problem. And I was wondering
how can you tell if there is air in the system or if the master
cylinder is out? I did a search on it, but found if the master
cylinder is at fault, it should be leaking right? and also
cause some pulse while you brake...abs? I told my mechanic
with out looking he said it was the master Cy!! He said there
is no way air could go in ( I suggest to him maybe air went in, if bleeding the brake with solve the problem?)
He is charging me 200 for the parts and 175 this
highway robbery?

There goes my paycheck!!
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Old 08-07-2002, 07:46 PM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
Air would not randomly enter the system. It would get in if the fluid ran low and air was pumped in or if the brakes were improperly bled. Also, if the brake fluid is really old, there could be mositure in the lines. If the fluid is old, then it needs to be bled anyway, you could try this first before the master cylinder is changed.
Ali Al-Chalabi

2001 CLK55
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Old 08-07-2002, 10:19 PM
csnow's Avatar
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Join Date: May 2002
Location: Mass
Posts: 1,127
Re: Help Brake question....Air or Master cylinder?

Originally posted by JOHNSC16V
if the master
cylinder is at fault, it should be leaking right? and also
cause some pulse while you brake...abs
Master cylinders would not typically leak externally when they fail, and there is no reason to expect any ABS pulsing.

A good indicator that your master is failing would be to press on the brake pedal for a while and find that it slowly loses pressure until the pedal hits the floor.

Water in the system would only be evident if there was fading under braking load [boiling], suchas repeated stops, or a downhill grade.

Air in the system would result in spongyness under all conditions, but is unlikely unless you ran out of fluid.

Having to push very hard to stop would indicate a brake booster failure. This symptom can be intermittent.

$375 is not outlandish for a Master. That's like 3 hours of labor.
1986 300E 5-Speed 240k mi.
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Old 08-08-2002, 02:48 PM
haasman's Avatar
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097
I didn't see what year, model or mileage on the car in question but with past cars I have hunted spongy brakes. I thought it was master cylinder problems.

I then realized the flexible rubber brake lines were shot.

I replaced all four, bled the system and the pedal was rock hard.

Have someone push on the brake pedal while holding the flexible rubber brake lines. You should not feel them expand noticeably.
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Old 08-08-2002, 05:49 PM
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Join Date: Jan 2001
Location: California
Posts: 2,068
If you're bleeding the brakes, make sure the ENTIRE brake fluid reservoir is TOPPED OFF so it looks almost as if it will overflow. There are two compartments in the reservoir. The rear half (which is a bit hard to see) feeds the rear brakes and only fills up when the front half starts to get really full.

Often, people bleed their brakes and wonder why they can't get fluid to come out of the rear calipers...
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