ABS actively resetting itself while braking (solved:master cylinder seal)

Beginning yesterday my abs started acting bizarrely. Still functions normal 80% of the time but under the right circumstances, such as extended smooth or light braking down to say, enter a roundabout or come to a stop behind the car in front of you at a light, upon final approach after nearly coming to a stop, the 'hard' point in the pedal decides to suddenly give way and reset nearly all the way to the floor. Which causes the car to suddenly lurch forward until your foot and brain catches up with the sunken pedal.

Makes it feel like I go from full pressure to air in the lines at the last minute. But seems to be the abs?

Any ideas?

Is the pedal kicking back at you like the ABS engages? Can you physically hear the ABS pump come on?

I can't say that I've ever actually heard the abs before but the physical sensation is as such
Need to slow down, apply pedal, pedal hardens as usual with full braking force after maybe 1/3 pedal travel. Then, after anywhere between 75-90% deceleration, hard pressure in the foot feed just kind of gasps and falls closer to the full travel of the pedal.

There's no aggressive abs pumping going on. Just normal smooth braking

I should note that braking is still smooth and effective once near the floor. It's just not cool having that moment of zero braking before it grabs again

Well, you would know if/when the ABS kicks on. It'll vibrate the hell outta your foot as your pushing on it. You literally feel the pulses in the pedal.

Have you tried to stop real hard (in a safe zone) and see what happens?

Yup! Operates perfectly well and pulls me to a hard firm stop.

I wonder if there's just air in the system. Maybe give the brakes a good bleed and see if that clears things up?

Yeah, I did do a full four corners caliper swap and bleed last month...I wonder if somehow some air finally made itself apparent after another 1k miles of driving.... Hmmmm

llamalor2112 wrote:

Yeah, I did do a full four corners caliper swap and bleed last month...I wonder if somehow some air finally made itself apparent after another 1k miles of driving.... Hmmmm

No. If there were air in the system, the pedal would be mush constantly, not just when coming to a stop.

If you can confirm that the ABS is engaging, I might guess that you have a Wheel Speed Sensor going out on you. Sometimes, when the air gap between the reluctor wheel and the sensor itself becomes too large, any speed under 7mph (estimate) would register a "0" to the EBCM, causing the ABS to activate on that wheel, and make it hard for you to stop. This happens a lot on trucks with the WSS attached to the knuckle and not the hub itself. I've seen it happen on cars like ours, but not very often. The only way to find out which wheel is acting up is to drive it with a scan tool and monitor all 4 wheel speed signals. It will not set a code because the difference in speed between all 4 wheels isn't great enough, and doesn't happen for long enough.

Why dont you just eliminate the Abs system altogether?

http://www.sjmmanufacturing.com/cam_brk_ls1.html

they don't make one for the Riv but SJM can guide you to a custom kit,kinda.

robotennis61 wrote:

Why dont you just eliminate the Abs system altogether?

http://www.sjmmanufacturing.com/cam_brk_ls1.html

they don't make one for the Riv but SJM can guide you to a custom kit,kinda.

Cuz the antilock has saved my car from both deer and a bear where I live. Kinda value it. And also not 100% that's the issue...

Scott, it will also replicate these results at higher speed than below ten. Say, slowing down from 60-- around as much as 25-30mph it'll dip the pedal as well. Still imagine it's a wheel speed sensor? That seems realistic.

They're integral to the hub as I recall? Only recent hub on the car is front passenger so that leaves a lot of possibilities from the others...if it's the case...I guess it could just as easily be that one. I'll make sure to check all the plugs and wired connections leading from them first thing tomorrow to begin with

You can rule out anything else by unplugging your ABS. drive around, if it behaves normally(minus the ABS and with an ABS light) then its one of your wheel speed sensors. I had the exact same symptoms on my old lesabre. I'd say Abaddon is spot on.

So realistically the best way for me to check which sensor is bum I should just check resistance at each of the sensor leads, no? See if there's consistency save for one?

llamalor2112 wrote:

So realistically the best way for me to check which sensor is bum I should just check resistance at each of the sensor leads, no? See if there's consistency save for one?

No. That isn't going to help you. I've already said how it needs to be checked....scan tool while driving monitoring all 4 Wheel Speed Sensors. That's the only way you're going to find an intermittent drop in wheel speed.

As Karma has suggested, you should pull the ABS fuse and drive the car. This will disable the ABS system and confirm whether or not the ABS is the problem. If the problem goes away after disabling the system, then you know that ABS is causing your problem. Then and only then will you take the car in and have it diagnosed (I assume you don't have a scan tool).

Proper assumption

I haven't used the dhp in a few years. Can it monitor wheel speed signal?

^would certainly be handy. I'm not sure what scanner for the 95 would check those parameters...

And fwiw, def not abs as removing the fuse did not alter the symptom

Forgot you had a 95.

I wouldn't blame the ABS. When the pedal gives way like that to the floor, that's a bad master cylinder.

LARRY70GS wrote:

I wouldn't blame the ABS.  When the pedal gives way like that to the floor, that's a  bad master cylinder.

And this was going to be my next suggestion once you ruled out the ABS.

Better fix it before it gets worse....the cup seals in the MC are getting soft. They roll over on themselves letting the fluid go right by them. They're going to get to a point where you won't be able to stop at all.

So definitely the master cylinder then and not wheel speed sensor now?

I'm currently on the road and set to do a solid 10 hours of driving today.....hooray

llamalor2112 wrote:

So definitely the master cylinder then and not wheel speed sensor now?

I'm currently on the road and set to do a solid 10 hours of driving today.....hooray

If pulling the ABS fuse and disabling the system made no difference in your condition, the only other reason your pedal would act that way is if the MC is failing. I wouldn't be driving for 10 hours with that car. There's going to come time when it will NOT stop. You're going to slam on the brake pedal, and it's going to go to the floor.

Do you feel a little "pop" in the pedal before it sinks?

It definitely gives a little bit of a sudden 'hey!' before it gives out so...yes. that's why I initially thought the kick was abs. Well.....crap... Amusing as it is I'm halfway to the local regional Buick meet right now. Maybe I'll have to bag it.

I wouldn't chance it....

True enough. Knew car had been running a little too well lately.... Thanks for your invaluable assistance and perhaps saving me from the unspeakable....yeesh

As a newb-level mechanic I'm curious though,  If it's mid braking and not at the start then I could see the seal caving under pressure but I feel like it would stay caved until released rather than caving then sealing again while under pressure? Just curious why it's re-setting itself somehow

Unless the seal itself is flexing under pressure and gapping I guess

Well, the seals on the MC piston are literally shaped like this [=====[ with the brackets being the seals, and the equals being the piston. When you push on the brake pedal, the piston moves forward, pushing fluid with it. When the seals get weak, they roll over backwards causing fluid to pass by them ]=====] This causes the mush brake pedal. When you let off the pedal, the seals roll back over into their proper form, "resetting" themselves. You may just have one bad seal, which is all it takes. Even if one of the seals is holding up just fine, you'll still have some decent braking. It's when they both take a dump is when you're in some serious braking trouble.

I hope you understand my keyboard drawing