Diagnos - Master cylinder on Chevy van

[Mrwurm]

Hey guys, I just picked up a used Chevy Astro van (1997 with ABS) that I plan to use to haul my ATV around. The van has 88,000 miles and the brake pads and components all look good. The brake pedal has a funny feel to it. It stops just fine, but the pedal is extra springy. Its like its always trying to push back against my foot. If I hit the brakes hard, the pedal really wants to 'bounce back.' I test drove several Astros before I bought this one and they did'nt have this feel. The pedal does'nt sink if I hold it, but it also does'nt feel as firm as it should. I'll try bleeding the system, but it really seems like its something else. Any thoughts ?

 

[Dusty]

To me, it sounds like the ABS pump is over charging the system. Try pulling the ABS fuse and take it for a drive. If all is normal, then it is in the ABS system.
Dusty

 

[TCBoomer]

On that vehicle, the only time the ABS pump operates is during an ABS event, unless the pumps relay is stuck on. It sounds like a wheel speed sensor problem. Does the brake pedal stop the "funny feel" when the vehicle is at a complete stop and the brakes applied?

 

[Mrwurm]

Thanks guys, I'll test the pedal at a stop tomorrow. I'll also try it with the fuse removed. Another useful test would be to find a safe area with a slippery spot and see if the ABS actually goes into action. (I'm not getting an ABS warning light on the dash)

 

[TCBoomer]

A condition does exists with the wheel speed sensors where a fault code may not set. To properly diag that situation, a scan tool would be needed to snapshot and plot the sensors individually. One of them is probably dropping out. It usually only happens at low speeds, under 10 mph, like driving through a parking lot. Does the pedal feel funny at higher speeds when braking?

Removing the fuse would disable the ABS...as Dusty said...it's a 60 Amp Maxi fuse located under the hood in the fuse/relay center.

 

[Mrwurm]

I'm not sure if it only happens at low speed. I've only driven it from the dealer to home. I'll test that out also. Will my OBDII reader tell me anything about this ?

 

[TCBoomer]

If what you have is just a code reader...no it won't help. You could use a volt meter that reads AC, like a Fluke or something similar. The Min/Max feature is what will record the event as long as the refresh rate is quick enough.

 

[RobS]

It could also be the power brake booster. Is it vacuum or hydraulic?

 

[TCBoomer]

It should be a hydro boost system which is why I asked if it does it all the time or just slow speeds.

 

[Mrwurm]

Hello folks, I drove the van with the fuse pulled for the ABS. Other than the ABS fault light on the dash, I did'nt notice the pedal feeling any differently. I've also tried some panic stops from about 40mph with the ABS enabled and without. The brakes are strong and effective. I'm beginning to think that the pedal feel may be normal for this vehicle. I've been driving Buicks for the last two years and maybe they just have a softer feel with less pedal effort.

To be safe, I'll have my local (trusted) mechanic give it a test drive when I have him inspect the front end components this week.

Thanks for your help.