It's not the ABS module, not on a '95. The 95-96 use the Bosch 5 ABS system, which is completely isolated from the primary brake hydraulics until an ABS event actually happens.

 

Good to hear about the ABS. Four new brake hoses ordered. Will post back results.
Thanks,
Nick

 

So you have tested and confirmed that the booster push rod isn't too long (Nomake Wan, post #2), and the calipers aren't sticking (me, post #16)? You never said that you did these tests.

 

Did not, but will do those tests as part of the process. After 25 years, I want to flush the brake fluid, and while we're doing that, might as well replace the 25 year old hoses too.
Thanks,
Nick

 

Why, oh why, do we try to help someone who will not listen???

Please don't expect any future help.

 

this coming from someone with reading and comprehension difficulty.

maybe he is right though. Someone broke into your car. Replaced the rod for the master cylinder and put it back together without you knowing. Hey...it could happen.

 

That's a bit harsh Roy. Sorry to hear you think it's a bad idea to flush the brake fluid and replace rubber hoses after 25 years. But, thank you for your input.
Nick

 

I believe that's insanely harsh.

I only suggested the pushrod as a possibility in case they had done something to the car in that area (brake booster, master cylinder) recently, or had recently acquired the car and did not know its history.

For me, the brakes dragged since I purchased the car. In addition, the previous owner mentioned replacing the brake booster. Thus, the first thing I checked was the push rod, which was fine. Next was the lines.

As always, best of luck, and it's absolutely a great idea to replace all four lines and flush that nasty old fluid.

 

i agree

 

Finally got around to working on this problem again. Here's an update on testing and results:
Without doing any further testing, we replaced the hoses with new, braided hosed and flushed the brake fluid. Brakes still dragged.

Next day, drove the car until the brake(s) dragged, jacked the right front wheel, could not turn it. Loosened the bolts on the MC a couple turns, maybe an 1/8" play, still could not turn the wheel. Loosened a few more turns. The wheel was able to turn some, but still very difficult. Took the MC all the way off. Wheel was a little easier to turn, but still dragging. I think just the fact that this all took time and things were cooling down. Put the MC back on all the way and the wheel was again, a little easier to turn. This all took about 30 minutes which is about how long it takes to free up w/o doing anything. Convinced it's not the pushrod.

Still not sure it's one wheel or all four so made it happen again, jacked the left front and could not turn it. Jacked the back up and was able to turn them both, but difficult and you could hear the pads dragging. So, all four wheels are affected.

Been going through section 5E2 (VIN P) to see if there are common points that would affect all four wheels. From what I can tell so far, the two lines from the MC go directly to the ABS unit and it splits off into the four wheels from there.

NOMAKE: Do you know the inner workings of the Bosch 5 ABS system?

Next step seems to be to pull the calipers. If I do that, I'm going to change the pads and rotors at the same time.
Thank you all,
Nick

 

Sadly no, not off the top of my head. I know the theories of operation, and there's a GM document regarding the internal workings of the Bosch 2U used in the 92-94, but the Bosch 5 no.

That being said, there's supposed to be an isolation valve that separates the Bosch 5 ABS unit from the brake lines. The ABS is not able to interact with the brake system until called upon by the brake control module, at which point the valves operate and connect the ABS to the lines going to the individual brake caliper hydraulic circuits. This is why you cannot bleed the Bosch 5 ABS without a Tech 1A or Tech 2; because you would need those to tell the brake controller to connect the brake circuits so you can bleed from each wheel.

 

Yes. I saw that in the service manual. It also said the ABS can not apply more pressure to the wheel cylinders than the MC is putting out. The MC is putting no pressure out when I have the car stopped in the garage. So I don't see how it could be the ABS unless there is something in there that can block the return of fluid to the MC??
Nick

 

Though the ABS on my '88 is far more primitive it turned out to be the culprit causing the right front to seize up and pull right on hard braking.

That was after I had replaced calipers, rotors, hoses and pads along with flushing the fluid.

I was ready to have the ABS removed from the "circuit" till a local shop found a working replacement.

 

Precisely. And if all four wheels are at play, that would probably rule out the individual wheel solenoids as there are two per wheel.

The Bosch 5 includes two valves used during ASR operation (a "Prime Valve" and a "Master Cylinder Isolation Valve" according to GM, or "Aspiration Valve" and "Switch-over Valve" according to Bosch) that internally prime the ASR circuit with pressure from the master cylinder reservoir, then shuts off the connection to the master cylinder to prevent the following ASR event from shoving pressure back into the master cylinder rather than it going to the rear wheel circuits. This is used for the individual brake control under 50 MPH.

Diagnostics for these valves is limited to electrical; and if you aren't getting any codes, that means the solenoids are electrically within their allowable ohm range and thus assumed to be good by the control computer. Whether they are actually good or not hydraulically, I'm not sure how to test that short of yanking the entire module and hooking it up on a bench with some sort of pressure testing apparatus and manually actuating the solenoid valves.

Your '88 uses a completely and utterly different system from the '95. So much so that it's not even worth discussing.

 

I had a friends car do this. After he did all the brakes,lines,fluid,rotors,pads TWICE I suggested to look at the wheel bearing. After he got the car nice and hot and wheel getting tighter we whipped off the front brakes completely and sure as s#$t, you couldn't spin the bearing. Let it cool and it was fine.

Also, just my .02. Good luck!

 

Good thought, but it happens on all four wheels, and I think the test I just did disproves that theory. I did about 40 miles of highway driving at 70-90. When I exited and stopped at the intersection, the car rolled fine. No dragging. Has to be in the calipers, pads, or rotors.
Nick

 

Wanted to give an update on my own issue.

Replaced the front calipers with new, front lines with new, front pads with new. All new hardware up front. Brakes still drag in the front once you get them hot enough. Will try yanking the caliper pins and lubricating them to see if that helps any. They're brand new, but I didn't do anything special to them.

At this point it does make me wonder what would happen if I were to yank the caliper straight off. I'm not sure how I'd do it with the wheel still installed but that would tell me pretty easily is there's a wheel bearing issue, I suppose.

As for the vibration issue I had mentioned earlier in the thread, it turns out that was a quirk of the tires I have. Nitto 555G2s are apparently infamous for flat-spotting after a very short time, and taking up to 20 miles of driving before going back into round.