brakes are thumpy... sometimes

[dorlow]

So, I have a 2004 Impala. It doesn't make sense to me why only sometimes my brakes are thumpy. sometimes when i stop, there is a definite pulse in my brakes and you can feel your body jerk as you stop. some days like today, coming from a 70ish to 0 but not slamming on the brakes... just easily slowing down, my brakes are as smooth as could be.

Here's the story, about 4 years ago, I redid my brakes. It seems like my brakes always get thumpy after so long. When I changed them 4 years ago, the previous brakes were about 1 year old, the factory brand new brakes, and they were thumpy. I went to a shop that all they do is fix and sell brake stuff and very reputable company in our area. I figured whatever they sell must be the best. I replaced my brakes with their stuff and the brakes were perfect. My brakes went about 2 years and were perfect the whole time. Never thumpy until about 2 years later. I figured that was a sign to change them again.

That was about a year ago. I went to the same shop and bought the same rotors and pads and changed them again. These are having the issue I described above. Some days there's absolutely perfect and some days they're very thumpy.

I'm not an expert, but from what I understand is the brakes get thumpy from the rotors getting warped (or maybe just sitting crooked.) When I work on my wheels, I always tighten them down with a torque wrench to make sure all the lugs have equal pressure to hold the rotor down square.

But if the rotors are warped, they should always be thumpy. The only thing I can think is if they're hot, they warp, but I just can't figure out how they'd cool off and go unwarped. It seems like once they warp, they wouldn't go back... at least not many times because my car does it all the time. It's been doing it for about 9 months... has its good days and its bad days.

[j cAT]

the brake calipers may have distorted pistons. I had pistons that failed as you are having your issues. the pistons get bell shaped. with new rotors and pads the piston is fully in the caliper. as the pads/rotors wear the pistons begin to bind up as they come out of the cal bore.

more cheap chinese crap. never seen this before. thanks GM , another buck you saved at my expense !

new calipers problem solved. NEW , NOT REBUILT...

Many rebuilt have this distorted piston.

[Tech II]

Jcat, never heard of that before....food for thought.....

Have seen a lot of problems with calipers, when only one is replaced, causing pulling to one side....problem turns out to be the piston size is not the same as original(maybe the rebuilt caliper is rebored and a larger piston is used).......with equal force being applied to different sized pistons, it causes a pull....

[j cAT]

Originally Posted by Tech II

Jcat, never heard of that before....food for thought.....

Have seen a lot of problems with calipers, when only one is replaced, causing pulling to one side....problem turns out to be the piston size is not the same as original(maybe the rebuilt caliper is rebored and a larger piston is used).......with equal force being applied to different sized pistons, it causes a pull....

I removed the pistons measured them worked them in and out of the cal bore by hand. so not guessing on what was what ...

my thinking is the abs activation pulses these soft pistons then they bow out in the middle hanging up on the weather seal.

both pistons the same damage distortions.

got new calipers poped out pistons checked for any defects they were perfect. bore had some dirt cleaned with some air then wiped down with alcohol.

brakes worked great still no problems.

[dorlow]

Originally Posted by j cAT

I removed the pistons measured them worked them in and out of the cal bore by hand. so not guessing on what was what ...

my thinking is the abs activation pulses these soft pistons then they bow out in the middle hanging up on the weather seal.

both pistons the same damage distortions.

got new calipers poped out pistons checked for any defects they were perfect. bore had some dirt cleaned with some air then wiped down with alcohol.

brakes worked great still no problems.

The ABS only activates if you slam on the brakes and the car can sense the wheels are going to lock up. 99% or more of the time, ABS doesn't do anything. Your car just works like a car that doesn't have ABS. Each hub has a sensor on it. Then it talks back to the master cylinder and causes it to releave pressure in the lines to let up on the brakes... but it's only if the brakes are about to lock up.

One thing I've never understood about the system though is how can the car tell the difference between the wheels locking up and the car just being at a stand still? Either way the car's wheels aren't moving. That I've never known.

For instance if you're going really slow on ice and then slam on the brakes, the ABS activates (you hear it working) and lets up on the brakes. But if you're going slow on dry ground and slam on the brakes, ABS doesn't activate. When you're on ice, how does your car know it's on ice and slipping instead of on just dry ground and the car not moving?

[j cAT]

Originally Posted by dorlow

The ABS only activates if you slam on the brakes and the car can sense the wheels are going to lock up. 99% or more of the time, ABS doesn't do anything. Your car just works like a car that doesn't have ABS. Each hub has a sensor on it. Then it talks back to the master cylinder and causes it to releave pressure in the lines to let up on the brakes... but it's only if the brakes are about to lock up.

One thing I've never understood about the system though is how can the car tell the difference between the wheels locking up and the car just being at a stand still? Either way the car's wheels aren't moving. That I've never known.

For instance if you're going really slow on ice and then slam on the brakes, the ABS activates (you hear it working) and lets up on the brakes. But if you're going slow on dry ground and slam on the brakes, ABS doesn't activate. When you're on ice, how does your car know it's on ice and slipping instead of on just dry ground and the car not moving?

I find your ABS activation really funny especially a guy/gal from the rust belt. slaming the brakes only causes ABS activation is a very funny statement.


you ever drive in ice,snow,wet leaves,sand/road salts ,very frost heaved roads. when appling the brakes the computer acts when it detects only a very small difference in the signal produced at the sensor. this is when the abs VALVES START BANGING AWAY AT THE CALIPER PISTONS.

driving in areas of no snow ,ice ,wet pavement, like a dry desert area with perfect roads, ya slaming the brakes, would be the only cause of NORMAL ABS ACTIVATION !

the ABS has a pump and valves. when it activates it releases and applies pressure to the brake system when you hold down the brake pedal. pumping the brakes with ABS is a no no...this is why holding the brake pedal down at the correct amount to slow the vehicle is the proper method to stop/slow down safely with an ABS system.

[oldblu65]

Originally Posted by dorlow

The ABS only activates if you slam on the brakes and the car can sense the wheels are going to lock up. 99% or more of the time, ABS doesn't do anything. Your car just works like a car that doesn't have ABS. Each hub has a sensor on it. Then it talks back to the master cylinder and causes it to relief pressure in the lines to let up on the brakes... but it's only if the brakes are about to lock up.

One thing I've never understood about the system though is how can the car tell the difference between the wheels locking up and the car just being at a stand still? Either way the car's wheels aren't moving. That I've never known.

For instance if you're going really slow on ice and then slam on the brakes, the ABS activates (you hear it working) and lets up on the brakes. But if you're going slow on dry ground and slam on the brakes, ABS doesn't activate. When you're on ice, how does your car know it's on ice and slipping instead of on just dry ground and the car not moving?

The way the ABS system works is it compares the speed of each individual wheel with all of the other wheels . Sitting at a red light , it senses no movement of any of the wheels therefore it isn't involved with the vehicle operation . When you are driving down the road , if one ( or more ) of your wheels stops turning ( as in the brakes locked up ) , the ABS senses it and releases enough of the brake line pressure to that wheel ( s ) to allow it to turn again thus stopping the lockup and allowing you to regain control of the vehicle . As for your statement on slamming the brakes on while on dry pavement and the ABS not activating - the ABS only activates if one ( or more ) of your brakes lock up . Slamming your brakes on dry pavement doesn't necessarily mean they will lock up and activate your ABS system . Hope this helps with your understanding of the ABS system ?

[trendkillerchris]

I have this same problem on my Impala 04 3.4 except my car strangly doesn't have abs on it

[maxwedge]

I believe the 04 Imp does have abs, check into that.

[trendkillerchris]

Mine doesn't

[maxwedge]

Ok, if you are getting a shudder when stopping ,no abs, the rotors could be warped, the most common reason for this.

[dorlow]

Originally Posted by maxwedge

Ok, if you are getting a shudder when stopping ,no abs, the rotors could be warped, the most common reason for this.

I replace my rotors with brand new ones every brake job. I don't bother resurfacing them.

[trendkillerchris]

I got the most expensive rotors you can get not even 2 months ago and it did little to nothing to solve the problem. They basically felt the same as the rotors I had before

[dorlow]

Originally Posted by trendkillerchris

I got the most expensive rotors you can get not even 2 months ago and it did little to nothing to solve the problem. They basically felt the same as the rotors I had before

The brakes I have on my car now are about 6 months old. I'm trying something new to see if it fixes my brakes going thumpy. The more expensive pads and rotors are harder to keep the wheels looking nicer. Harder pads give off less brake dust. But also harder brake pads the hotter they get when stopping which leads to the rotors getting hot then they warp.

I bought the cheapest brake pads because they're the non metallic which means they'll break down easier and stop nicer but more brake dust and will need replaced sooner. My rears are semi metallic because they didn't sell non metallic ones for the back.

[trendkillerchris]

So what do you suggust? Buy cheap brake pads?