I am having a really strange problem with my brakes. I have a 99 Limited with ABS. I have noticed 2 different scenarios when this happens. First, If I encounter loose gravel on top of pavement, and I am braking normally at an intersection. My brakes feel like they give out and I get this back pressure that 'fights' my foot up and down until I completely stop, then it is fine. Second, I have this one intersection near my house that is sloped about 20% right to left...like this /. The road continues but my road is to the right in the middle of the turn. When i go to turn right at the intersection, essentially going over a hump because the road drops off on the other side. more often than not I get the same occurance. The right front tire never leaves the pavement but the weight is shifted to the left substantially. It feels like the pressure is reversed and i get this back pressure that makes the brakes shudder. If that makes sense. I can feel the brake pedal pulsating backwards and at that time, there are no brakes for a few seconds. Kinda scary. I had this happen on slightly wet pavement one other time while stopping at an intersection. It feels like the ABS doesnt engage. I have replaced the calipers, pads, rotors, brake lines and had the brakes bled numerous times. Things I am considering are master cylinder, brake booster, and possibly some kind of vacuum problem. Anyone experienced this strange problem?

 

I have had the same thing happen. I think the shuddering IS the ABS kicking in. But thats just a guess. When I am stopping and hit a bump or wet roads I think my ABS kicks in a bit early and does what you described. If anyone has any answers to this that would be great.

 

Yeah, your descriptions sound like normal ABS operation to me. Your post count is pretty high so I am betting that you know how ABS works but the system will vibrate the pedal to let you know that it is reducing the amount of brake being applied to keep the wheels from locking up.


This is a little off topic but people complain that ABS sucks and locking up the wheels will stop them faster. While this may or may not be true, it is true that you cannot change direction while your wheels are not rolling at all. That is the beauty of ABS, it will at least give you chance to steer clear of any obstacle in your way.

 

Yup, sounds like ABS to me. Scares the crap out of me every time

 

Well I guess. It just seems squirly to me. I guess the ABS is freaking out because there is nothing for the tires to grab onto while traversing the gravel and it applies the back pressure to comensate for that?

 

same thing happened to me last year when hunting...i was goin about 55 on a paved road and needed to turn around, so when i had slowed to about 15-20, i eased off the road while mashing the brakes, and once i hit the loose gravel on the roadside, it felt like it was shuddering...but good to know it's normal operation of the ABS, as it kinda scared me... lol!

 

From what you have describe you have perfectly working ABS system. There is nothing wrong: you do not have a strange brake problem :-)

 

I agree, ABS acts by pulsating during braking to avoid locking up the tires. It measure the speed of tire rotations and compares them to each other during braking. When one wheel slows too much compared to the others the system starts kicking in. Sorry you had changed a lot of your braking system just to find it behaving normal.

The purpose of this is to allow the driver to maintain steering control under heavy braking and, in some situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control). Disadvantages of the system include increased braking distances under certain conditions and the creation of a "false sense of security" among drivers who do not understand the operation and limitations of ABS.

from: http://en.wikipedia.org/wiki/Anti-lock_braking_system

 

Basically yeah...the ABS system has to interrupt the brake pressure in between the pedal and the caliper, effectively undoing the brake on that wheel for a split-second, ideally to let the wheel stop slipping and come back up to vehicle speed, then it tries the brake again..hence the pulsing.

Sometimes the ABS gets overly aggressive, not sure how common this is on 4runners, but on Land Cruiser's it is common to go over a speed-bump or pothole and be gently on the brake and the ABS goes nuts and then you freak out because the vehicle doesn't want to stop. Taking the ABS sensors out and cleaning them off (they are a magnet and will gather up any metal shavings/pieces wandering around in the birfield grease)...however, that of course is only for a Land Cruiser. Though I guess the 4runner ABS sensors could have some random metal bits on them, though I don't know where they would come from.

My 4runner has done something similar, and on a slushy road it's a bit of a pain, it does help to just push the brake pedal down harder, at least in my experience...still pulses, but the vehicle stops better/quicker than trying to ease up on the brake pedal pressure.

 

Try sliding down a muddy hill when 4wheeling when the ABS kicks in, things get hairy then!

 

haha, yeah...2" of rain in a hour + 1/2 mile of downhill clay logging road + ABS = one heck of a wheelin experience...

 

Should be a simple fuse you can pull to disable it. Course ABS light comes on..

 

Yeah I did that once and decided it was better to leave it in because it still slid downhill and locked up without the pulsing.

 

Never drove a non abs vehicle in those kinda conditions? Pump the pedal..

 

Yes I realize that, its more of a habit when you've driven with an ABS equipped vehicle than non.

 

Know what ya mean, tbh I am used to it since I drive alot of different vehicles day to day.

 

Thanks for the help guys! I really appreciate it!!!