ruh ohh whered the brakes go?

91 rmw new rotors pads shoes spring kit andwheel cyls. On rough pavemant while applying the brake occas on hard stops and if I am at a light and applying the brake enough to let the pads touch and drag the pedal drops and vibrates. Once I let go and apply again its fine, but the abs light comes on. Ive heard two possibilities one is the master failing, the other sounds more likely and its needing the brakes powerbled. Any other suggestions or agrees would be appreciated.

Well, since bleeding the lines would be easy and cheap. I'm sure that's where you are planning to start. Good idea.

Dave

Check the fluid level in the master cylinder as well.

I would look closely at the booster vacuum line coming from the engine.About 3 weeks ago on my brothers 99 tahoe,brake pedal would go soft on.Replaced line and no more problems.

poppawoodie wrote:

91 rmw new rotors pads shoes spring kit and wheel cyls. On rough pavement while applying the brake occas on hard stops

both situations that could potentially trigger the ABS system

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and if I am at a light and applying the brake enough to let the pads touch and drag the pedal drops and vibrates.

The ABS motor on these cars makes a very noticeable vibration in the pedal. Usually quite audible too. However, the ABS system is not supposed to operate at very low speed -- 4 mph, IIRC -- because the wheel sensors are not sufficiently accurate.

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Once I let go and apply again its fine, but the abs light comes on.

ABS light = problem with ABS system. Does the light stay on or go out when you release the brakes? (It should actually latch on until the key is cycled).

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Ive heard two possibilities one is the master failing, the other sounds more likely and its needing the brakes powerbled.

Final pedal height changes when brakes are pumped (pressed and released once or twice) -- but pedal stays firm at final height -- usually indicates a small air bubble in the system. If the pedal slowly continues to drop, that indicates a leak in the system somewhere. If the leak is not external and there is no fluid loss over time, then the leak may be internal to the master cylinder. Neither an air bubble nor a leak should cause vibration in the pedal.

It never hurts to bleed the brakes. In fact, you should suck the fluid out of the reservoir, wipe it out and bleed the brakes every couple of years just to flush out the accumulated dark spooge. So go ahead and start with that. But I think you need to scan the ABS computer and find out what's going on in that tiny little brain.

It should be setting an error code when the ABS light comes on but this may be a case were the raw data is more important. Specifically, what are the wheel speeds at the time? How do the wheel speeds compare to the vehicle speed sensor? Sitting at a light, all 4 wheels should be 0 -- and therefore the ABS system should not be trying to operate (assuming that's the source of the vibration you feel in the pedal). IIRC, the ABS compares the wheel speed signals against the vehicle speed signal from the transmission to determine which wheels are losing traction (ie: significantly slower). Something strange is going on if the ABS is kicking in at a stop.

I had the proportioning valve stick with total bias to the rear brakes. The only remedy was a used one from a junk yard. When the brakes were applied, they were not effective to a great degree. I seemed like I had "poor" brakes. I did have an accident because of it. The car would not stop, and I had the brake pedal pushed all the way to the floor.

Try cleaning all the metal and crud off the front abs sensors. I had the exact same problem with my gmc sierra and my dad's ford ranger. Faulty front abs sensors cycled the abs at parking lot speeds, just before i came to a stop. Made for quite a few butt-puckers when the truck wouldn't want to stop pulling into a parking space or behind a car at a set of lights!  

Thanks guys my left foot has been permanently stuck to the e brake just in case, I will check out your suggestions. Its only at parking lot speeds, sensors sound pretty likely at this point. Its a def failure to stop and ive panicked a few times, I will let you know what happens

Unplug the ABS fuse to disable the system until you get this figured out and fixed. If this is an ABS system failure, that should allow you to stop again.

buickwagon wrote:

Unplug the ABS fuse to disable the system until you get this figured out and fixed. If this is an ABS system failure, that should allow you to stop again.

Now thats an educated man thank you, sometimes I need a slap across the back of my head to wake me up. Pullin fuse first thing in morning

Well had system flushed and power bled so far so good, hitting rough roads and brakin is minorly sketchy but wayyyyy better

Dont forget to reinstall the ABS fuse when the brakes are finally functioning correctly.