Low speed brake failure

[goser]

I've been experiencing reduced stopping power only when coming to a slow stop (like moving ahead in drive-through speeds). It manifests as a jerking of the vehicle as it moves forward an extra few feet before coming to a sudden stop. At first I thought it was the ABS kicking in but it happened again today on dry roads. I don't see anything obviously wrong--lots of pad left, the rear calipers and rotors are only a year and a half old. The front pads are about three years old and the rotors have at least 60,000 miles on them. I don't have any way of checking their thickness but they appear to have lots of meat left. The front calipers are just as old, but pins move freely and there are no signs of uneven wear. Are these symptoms indicitive of glazed pads or worn rotors? Or should I be looking into an abs sensor failure? Any suggestions on how to troubleshoot would be appreciated.

[Chris Stewart]

Does it act like you hit the gas or suddenly seem to apply lots of brake with no extra brake application on your part?

Do you have a Check Engine or ABS light?

[goser]

No check engine or abs light. Stepping on the brake it slows normally but then just as it should come to a stop it feels like the abs kicks in. The last time it happened I stood on the brake which, although the pedal was firm, had no effect.

I'm just realizing as I write this, could there be some kind of a drop in vaccuum to the booster that's occuring as I approach idle? I have noticed the past few months that it sometimes bogs down when I first put it into gear or switch between drive and reverse, but I hadn't given it much thought at the time.

[drdd]

a little bit of a stab-in-the-dark, but ...

do you have any check engine lights or codes about your VSS? (Vehicle Speed Sensor). I'm pretty sure that '98's have these, but not 100% certain. They're usually connected to your transmission (again, not 100% certain ... maybe some others will chime in ...). Might also explain the bogging down when switching gears ... ???

Originally Posted by goser

No check engine or abs light. Stepping on the brake it slows normally but then just as it should come to a stop it feels like the abs kicks in. The last time it happened I stood on the brake which, although the pedal was firm, had no effect.

I'm just realizing as I write this, could there be some kind of a drop in vaccuum to the booster that's occuring as I approach idle? I have noticed the past few months that it sometimes bogs down when I first put it into gear or switch between drive and reverse, but I hadn't given it much thought at the time.

[MT-2500]

Originally Posted by goser

I've been experiencing reduced stopping power only when coming to a slow stop (like moving ahead in drive-through speeds). It manifests as a jerking of the vehicle as it moves forward an extra few feet before coming to a sudden stop. At first I thought it was the ABS kicking in but it happened again today on dry roads. I don't see anything obviously wrong--lots of pad left, the rear calipers and rotors are only a year and a half old. The front pads are about three years old and the rotors have at least 60,000 miles on them. I don't have any way of checking their thickness but they appear to have lots of meat left. The front calipers are just as old, but pins move freely and there are no signs of uneven wear. Are these symptoms indicitive of glazed pads or worn rotors? Or should I be looking into an abs sensor failure? Any suggestions on how to troubleshoot would be appreciated.

Your ABS system is kicking in at low speeds.
Usually caused by speed sensor problems.

You need to watch your speed sensor reading on a scanner and make sure all wheel speeds match.
Keep tires matched and proper inflation.

On your jimmy and some other GM's there is several TSB's for that problem.
One is.
02-05-25-006A
Pertains to cleaning rust built up under front wheel speed sensor mounting.
Check and clean them.
Let us know how it goes.
If that does not help post back.
Good Luck

[goser]

Thanks for the response MT. I found the TSB on another blazer forum, and although I could not get either side to read above 350mV, the side that I recently replaced gets up to 204mV but the other side only gets up to about 130mV.

[MT-2500]

Originally Posted by goser

Thanks for the response MT. I found the TSB on another blazer forum, and although I could not get either side to read above 350mV, the side that I recently replaced gets up to 204mV but the other side only gets up to about 130mV.

You are welcome.
It takes a scanner reading to test them out good and make sure wheel speeds match..
But if you have replaced one side you might as well replace the other side to match it.

Be sure to clean off all rust build up on sensor mounting.
Good luck and let us know how it goes.