My '98 has 112k miles and I've developed a new brake issue. I replaced the rear calipers about a year ago and they're fine. I recently developed a problem where (sometimes, not always) in the last second or two of normal braking the brake pedal suddenly drops a bit and there's a 'clunk' in the brake system. I thought it was a caliper piston hanging up so I ordered and just finished putting in 4 new rotors, all new pads and new front calipers. The car stops on a dime...but the clunk and pedal drop still happens maybe ever second or third stop.
I've read similar threads where this may be an ABS issue but it wasn't confirmed that this was the cause. I guess it could be a master cylinder but I can't be certain.
Any further ideas? If ABS, what do I look for? Can the sensors be cleaned or should they be replaced? Is there a way to test the sensors? Electrical continuity or resistance check?

Thanks in advance for your help. This forum has been a great source for us shade tree mechanics!

 

If this was an ABS sensor issue then it would be popping up an ABS light when the problem occurs.

You'll need to track down the source of the clunking noise.
Fire up the truck in the driveway and hit the brakes and hold them a few times and see if you can get it to "clunk" again when not moving.

If it does then grab a buddy and have one of you listen at the master cylinder while the other works the brakes. More then likely you will find that the noise is coming from there.

Before you go and replace it though, be sure to check the vacuum line that goes to it. Your problem could be as simple as a rotten or collapsing hose.

If your noise doesn't come from the master cyl then where it does come from will give you a clue as to what the issue is.

 

Another way to test the master cylinder is to leave the vehicle off and push the brake pedal down and see how fast it stiffens up. I think if it becomes hard to push by the third pump it the cylinder is good, but that doesn't mean that the vacuum hoses are. I was just researching things on wheel bearings for a 2000 blazer and found out that the ABS sensor is in the hub and causes issues when the bearings go out. This is what some one said "sounds like something similar to what i had to fix. if you get problems with your ABS light and are able to turn it off by shaking the wheel left to right or pull into a parking spot and the brake drops to the floor, its most likely your hub assembly. its a pain in the butt, and they usually both go at the same time." This describes to a T what mine is doing and just now started getting the wheel bearing noise. I pulled the fuse to fix the pedal drop problem. Hope this helps.

 

Thanks guys for the great replies! The current update is that the problem seems to have stopped, at least for now. I went on a local drive yesterday and didn't sense the problem. I have searched the ABS wheel sensor and these can be checked using a volt/ohm meter.
You may be right about the vacuum issue too. I need to check that out. Sometimes when I'm in a rush I start the car and quickly try to shift from Park to Drive the tranny doesn't fully engage...so I'm thinking the engagement or trans itself may be vacuum assisted (?). If I wait 2 seconds before shifting the tranny engages just fine. (The fluid and filter were changed about 6 months ago).

My manual is lost as a result of our recent move so I'm probably asking stupid questions, so again I thank you for your patience and adivce!

 

Not sure if your problem stayed fixed, I have a 98, been fighting the ABS sensors for a while (new sensors etc) yesterday I tried what a Chvey service bulliten said was the issue... rust develops under the front wheel sensor mounting plates.... sure enough when I removed the moutning plate and cleaned out the rust the ABS no olonger inappropriately fired during slow stops... so it makes sense that new hubs would fix the issue too ...

 

rsher, Thanks for the update. No, my problem didn't stay "fixed". It's back and worse than ever. Today I pulled off one front wheel and the caliper and disc to get to the sensor. I also disconnected the ABS sensor line from the harness. Across the two wires I was getting about 3000 ohms resistance which is within spec. With the sensor connected you are supposed to get some voltage when you spin the wheel. Either I didn't spin it fast enough or it isn't working right, but since the ohm reading is within spec I thought it was OK. I also noticed the hub end of the sensor was very dirty and greasy so I cleaned it. I then put it all back together and it's worse than ever! I just returned from a short trip and the ABS enaged at every stop. At one stop the ABS pump kicked in and STAYED ON until I tapped the brakes again.
By the way, I'm not getting the ABS light on the dashboard when this happens, but I do see it when I first start the car. Is this a problem?
I did notice a little dirt and rust at the mount on the hub but maybe I didn't clean it well enough. Tomorrow is another day and I'll take another look at it then and also do the other wheel.
Thanks for the heads up...and I'll keep this thread updates with results.

 

There should not be dirt and grease inside the hub assembly in the ABS sensor area. If you do, then it usually means that the bearing is going bad and the grease on the ABS sensor ring has toasted your sensor.

Replacement of the hub and sensor is usually your only option at this point. You can disconnect the sensor and pull the ABS fuse to disable the system until you get it replaced as having your ABS going off like that could be very dangerous.

 

Today I checked the hubs, cleaned them as best I could, cleaned the sensors and double checked for ohms. My test drive proved the ABS is still engaging at every stop. I hear no noise from either front hub so I am not sure the problem lies there. I will search this forum again for additional ABS info, ECM, ABS pump, etc.

awe341, that's good advice. So, I'll pull the fuse until I can figure this mess out. Thanks again for everyone's help.

 

Just a quick reply. I searched extensively on this forum and have found similar but not exactly the same sort of issue. I also read the Tech Article in the DYI section regarding low speed ABS activation but the fix only applies to the earlier (1st) series Blazer (I think). On the '98 4x4 front hubs it appears the air gap cannot be adjusted as there is a stainless plate that is riveted to the hub that the sensor end goes through. I'm not sure what the purpose of this is but I am reluctant to remove it. With the ABS sensor removed (and the hole plugged to keep debris out) I cleaned the surfaces of the sensor and the stainless plate and surrounding areas with a brass wire brush and brake cleaner. When all reassembled the problem remains...the ABS engages at low speed during every stop.
As stated above the hubs appear to be ok...I can't hear any noise coming from them and there's no play in them when I try to shake them with the rims mounted. So, I welcome your ideas and suggestions. Thanks in advance for your help!