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Old 07-21-2002, 01:58 PM   #1
Angry Joe
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Default Loss of brake pedal pressure, no fluid leakage!

I noticed this problem when when I was in line for my last autocross run of the day, which scrubbed my run. I noticed a disconcerting loss of brake pedal pressure. It does not occur every time I hit the pedal, but it did happen frequently on the way home. I always had enough pressure to stop safely, but the feeling is disconcerting to say the least.

There is no sign of brake fluid leakage and my fluid level is fine, so I'm at a loss! I did not notice a problem prior to or during my autocross runs.
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Old 07-21-2002, 02:39 PM   #2
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

it could be the master cylinder. check inside the car behind the brake pedal where the post goes through the firewall to the brake booster. your carpet will be wet if it is leaking inside. if your master cylinder is leaking, or if it is bad then you will have a very similar problem like you are discribing.
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Old 07-21-2002, 02:43 PM   #3
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

It's your brake booster! The brake booster uses vacuum from the engine to multiply the force that your foot applies to the master cylinder.

The vacuum booster is a metal canister that contains a clever valve and a diaphragm. A rod going through the center of the canister connects to the master cylinder's piston on one side and to the pedal linkage on the other.

Click the image to open in full size.

Check out www.howstuffworks.com

Some of my friends have had the same problems and replacing the brake booster solved the problem..
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Old 07-21-2002, 03:05 PM   #4
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

Great feedback! Am I right in assuming it's a bad idea to drive the car? Looks like I'm hitching a ride to work tomorrow Click the image to open in full size.
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Old 07-21-2002, 04:15 PM   #5
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

I actually had this problem in my '93 civic... noticed it after I bled the brakes.
I assume what you're describing is as follows...
-coming to a stop... brake normally... okay...
-coming to the next stop... start pressing brakes... oh sh! the only way to maintain pressure is to keep depressing on the pedal.... further and further.
-next stop... hmm... brake pressure seems close to normal now
I noticed this especially when it was hot out, or the car had been sitting in the sun for a long time.
I tried those brake test things listed in the Helm's manual, none of them directly corresponded to what was happening. In the end we just replaced the master cylinder and the brakes seem okay now.
I suspect an internal leak in the master cylinder. No fluid is leaking out, but fluid leaks past the internal seals... I guess in the heat the fluid gets less viscous?
That's all my theory anyway...

edit: i suspected the booster too, but the problem is such that it actually feels like fluid is just slipping past the seals in the cylinder. Like if you imagine it as such, it makes perfect sense when it's happening. Sorry if this doesn't help tho.


[Modified by nsadhal, 5:17 PM 7/21/2002]
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Old 07-21-2002, 04:25 PM   #6
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Default Re: Loss of brake pedal pressure, no fluid leakage! (nsadhal)

That does sound very much like what was happening, although sometimes the pedal sank on the first push and releasing it restored pressure. More often though the behavior was the opposite.

It was pretty hot out and the car was in the sun all day, but obviously my abuse of the brakes (either excessive pressure, excessive heat or both) instigated the problem. I'll see if the problem reoccurs with the same severity tomorrow morning.


[Modified by Angry Joe, 1:29 AM 7/22/2002]
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Old 07-21-2002, 07:13 PM   #7
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

Sounds like a "fading master" to me. Hold steady pedal pressure and the pedal fades? First check all lines and fittings for any obviuos leaks ( rear of master, wheels cyls., calipers, and hoses) If everything seems ok it could be the master cyl. leaking internally (fluid bypassing internal o rings and seals) Click the image to open in full size.
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Old 07-21-2002, 09:36 PM   #8
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Default Re: Loss of brake pedal pressure, no fluid leakage! (samiam)

I did a rear disk conversion on my EK a couple months ago and i went through 5 master cylinders. eachone had its own problem. I experienced the problem that you are having now. It is the master cylindr. Get a new on from Honda , no were else, must be OEM and makesre u bench bleed it.
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Old 07-24-2002, 05:17 AM   #9
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Default Re: Loss of brake pedal pressure, no fluid leakage! (Angry Joe)

Problem was indeed the master cylinder. Thanks for the input Click the image to open in full size.
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