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I am trying to help a buddy of mine fix a brake issue he is having on his '91 DeVille.

The fluid level was low, so we added some and then when that didn't help, we bled the brakes. I am not familier with the car so I was unable to tell when I drove it, but we were able to narrow down the problem. The brakes work great in a stop and go situation. The problem is, at high speeds, when you would gradually apply pressure to slow down, the pressure bleeds off until you are to the floor with little braking power.

Here is what I can tell you for sure:

1) when the car is off, you can build up good pressure when pumping the brakes.

2) When traveling under 30 or so and if you jam the brakesm it stops you as expected.

3) when running, if you keep your foot on the brake, ir apply pressure slowly, the pressure bleeds off.

4) not losing any fluid that we can tell. the underside of the car is pretty clean and its not leaving puddles.

Can anyone help? He's not in a position to take it to a mechanic or we would and be through with it.
 

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My first guess would be the brake booster/line has a small leak...... Just enough so that vacuum will slowly escape....... Someone else will chime in to confirm/or say something else!!! As I am not sure......

And, welcome!!! :wave: Thanks for signing up!!!
 

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It sounds like you have a master cylinder problem the reason you don't see fluid leaking is because it could be leaking right into the booster.
Check that out. Brake fluid is not good inside of the booster, it will mess up the diaphram in there...
 

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Discussion Starter #6
>>If you pump the brakes and hold the pedal down, does it sink?<<

When the car is running yes, when its off, the pressure builds up and stays.


>>It sounds like you have a master cylinder problem the reason you don't see fluid leaking is because it could be leaking right into the booster.<<

We'll take a look, however, the fluid level isn't dropping, would this condition cause it to significantly lower the fluid in the reservoir? I mean to a noticeable point in a short time?
 

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Hi,

What was the resolution of this issue? My 92 Deville is doing the same. The confusing part about the master cyl diagnostic is the engine on/off condition. Wouldn't a leaking master cyl also drop the pedal when the engine is off? Mine is similar as original post: when the engine is on, steady pressure will drop the pedal - but fast press works w/o issue.

Another related (?) issue - the Antilock light will illuminate (but the brake light is off). Seems to happen intermittently - but once lit - stays lit until key off. Is there any way to read the codes with the Teves mark 4 ABS system? I enjoy being able to access PCM/BCM stuff thru the climate control system - but the ABS doesn't seem to talk to it.

Thanks in advance!!

-martin
 

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when the engine is on, steady pressure will drop the pedal - but fast press works w/o issue.
R & R the master cylinder.

Another related (?) issue - the Antilock light will illuminate (but the brake light is off). Seems to happen intermittently - but once lit - stays lit until key off
Most likely a bad wheel speed sensor. The bearing will start to growl in a few months.

Is there any way to read the codes with the Teves mark 4 ABS system?
The ABS codes can only be read from a module under the passenger seat if I remember correctly. I think you have to use a Tech II. It does not communicate with the onboard diagnostics.

I had both of these problems on my '92.
 

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Thanks Ranger! The replacement master cylinder did the trick. No more sinking pedal! I did notice that my Antilock light came back on after a few test 'panic' stops to test brakes. I pulled over to the curb, shut off the engine for 10sec and started it up again. The ABS light was off and stayed off - even during 'moderate' braking tests.

Any ideas? I hate the idea of paying Brakemax $80 to plug into the ABS computer and give me the code... *sigh*

-Martin
 

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If the ABS light is not on, don't worry about it. If it comes back on, run a continuity test on all 4 sensors at the back side of each wheel hub. The one with no continuity has a bad sensor.
 
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