abs seems to work but can not stop

[rgmc]

had my rotors turned installed ceramic pads
2 weeks later coming down a hill the abs activated and I had to push hard to stop.Thought it was just wet. Couple of days later I pulled up my steep driveway, when I braked at the top the abs activated and I crashed into my garage.kind of hard to explain to the wife. The van is parked. when I brake sitting in it the pedal is firm but I can push it it down pretty far and the abs seams to activate. cleaned the sensors and rotors with brake cleaner. I had the abs light come opn for a second some time ago.everyone is still laughing.

[old_master]

The master cylinder may have an internal leak, (seals worn). When you push the pedal, it should be firm and hold. If it sinks to the floor, there is a leak. Check for external, visible leaks, if none, it's probably the master cylinder.

[rgmc]

Thanks Master
Van still sitting,garage almost repaired
I donot see any external leaks
I pulled the abs fuse and with the engine running the brakes still go firmly
to the floor and there is a sound similar to the sound you hear when you turn
the wheel to far. Had the power steering replaced once and it has always
made noise (wining).
Thanks again
rgmc

[rgmc]

OK
never mind about the hissing noise
it would seem that it is a normal brake booster noise
(hayes manual) So I guess the master cylinder it next.
I always think of an old cartoon
Thanks
Again

[rgmc]

OK
Changed master cylinder
Rear brake cylinders
Front calipers
Bled everything.
Good pad preasure
When I put the ABS fuse in the ABS still activates at about 3 mph.
CanI retreive a cod eor something?
Thanks
RGMC

[CD Smalley]

Have you changed tires to some that are over/undersized?

If so that could cause the ABS's calculations to be off.

[old_master]

Two things to keep in mind while servicing ABS brakes: First, when retracting the caliper piston, the flex line needs to be blocked, and the bleeder opened to prevent impurities in the caliper from being pushed back into the ABS unit, (the solenoids are very sensitive to dirt). The alternative is to remove the flex line from the caliper before retracting the piston. Second, if the ABS unit runs dry, even momentarily, from a leak, or during bleeding, it is mandatory to use an ABS capable scan tool to bleed the system, no way around it. Gravity bleeding or power bleeding will not work. The scan tool manually operates solenoids within the ABS unit to allow complete bleeding of the ABS unit. ABS capable scan tools are available, but they're expensive. Make sure it's capable of operating the solenoids before you buy one. Either one of those conditions could explain the two week time frame you mentioned in your first post, or it could be coincidental, and something totally different is wrong. ...just a couple of possibilities.

[hog buff]

a worn bearing or abs sensor with a build up of debree (metal or rust) can cause false low speed abs activation.a scan tool with abs data can be used to see if any sensor reads diffrent than the others

[ericn1300]

General Motors Corp. said it is recalling about 800,000 sport utility vehicles and pickup trucks in 14 northern states because corrosion was affecting the antilock brake system, leading to more than 200 low-speed crashes.

From: http://www.aa1car.com/library/gm_suv_recall.htm

Links to the NHTSA recall database there

If you want to fix it yourself:

The symptom of this problem is low-speed ABS activation with no fault code stored in the powertrain control module. The ABS engagement is generated by a slightly too-large air gap between the sensor head and reluctor ring on the wheel hub caused by corrosion and a buildup of debris on the sensor mount.

The repair is relatively straightforward.

First, confirm the problem by disconnecting the wheel speed sensor from its harness, connecting a voltmeter to the sensor wires and rotating the wheel by hand at least one revolution per second. Measure the voltage generated by the sensor -- it should be at least 350 AC millivolts. If the voltage is low, remove the brake caliper and rotor, unbolt and carefully remove the wheel speed sensor and thoroughly clean the sensor's mounting surface on the hub. Apply a rust inhibitor/lubricant and remount the sensor (if it is still serviceable). Full details of this service are available online at most local public libraries through the Alldata database.

from: http://www.startribune.com/cars/11354966.html