'94 Sonoma Brake Problem

[94sonoma]

What is happening with the braking system. If I'm traveling over a bumpy terrain, and my suspension is being compromised, i.e., (uneven travel of the front tires going up and down), when I push the brake pedal while traveling over a bumpy terrain, the brakes will start to grab in their ABS mold, but a split second later, you can feel the brake pedal starts to pulsate, you can feel the brakes let loose under your foot and the ABS system just seems to release while the brake pedal will suddenly go towards the floor in the next second. Now it doesn't go all the way to the floor completely, (maybe a couple of inches away from the floor, and I'm stretching that only because I have not measured the distance), but if your applying that HARD BRAKING FORCE on the brake pedal, the ABS stops working and the vehicle is not stopping as it should. It more or less starts to glide. This can happen at any speed. It's a feeling like the brakes just stopped working.

Now if I'm on a relatively smooth surface, the ABS system works just fine and none of the above explanation happens.

One other note; brakes and fluid were changed about ten months ago. No fluid leaks then, no fluid leaks now and the master cylinder reservoir is topped off.

Thanks,

Darryl
'94 Sonoma

 

[lmleck]

The master cylinder seals may be leaking internally during abs pulsing...
Gently push and then bounce the brake pedal with your foot, it may begin to head for the floor.

 

[94sonoma]

I'll check that, but I'm not seeing any leakage around the master cylinder. I've read in a Chilton's repair book that it may be faulty speed sensors at the wheel. I'll try what you have suggested and report back.

Thank You,

Darryl

 

[mogleybear]

I have experienced this in other vehicles. I believe that it is caused by the tires losing traction and gaining traction as the load on the individual tire changes. I had one particular part of an intersection in my town that would cause this sensation in every vehicle I drove that had anti-lock brakes. It is difficult for the computer to make the adjustments to the pedal force when the tires have so much change in traction. Imagine a washboard road, as the tire hits the bump part, it has a higher load on it, more braking force is required to stop the tire. As the tire goes into the valley part of the washboard, it has a lot less load and a lot less braking force required to stop the tire. Because of this you get the changes in the pedal feel and braking action. Basically the ABS computer can't tell if it is skidding or not.