1990 Poor Brake Performance
Melting Slicks
Joined: Jul 2005
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I’d always seen people say to start at the farthest caliper & work toward closest caliper to the master cylinder. However a retired sae master mechanic friend told me to do it the other way. He had his specific reasoning so I do it both ways just to be sure. It’s overkill, but I have lost brakes (not on a vette) & it wasn’t fun!
IMHO - it really doesn't matter which wheel you bleed first in an ABS car. Just flush fluid thru until you get nearly clear fluid from each caliper.
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Instructor
Joined: Jul 2020
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From: New Jersey
Have you checked the Power Brake Booster for proper functioning ???
Start the car with your foot lightly on the brake pedal - the pedal should move down a bit when the engine lights up, and generates vacuum.
Try disconnecting (and plugging) the vacuum line to the power brake booster - the power assist should be gone after a couple of brake applications. Does the braking with no power assist get much worse, or is it comparable ??? Obviously - it should be quite significantly worse.
BTW - Have you tried the old 2 person bleed technique, where one person depresses the brake pedal while the other opens the bleed valve, and closes the bleeder before the brake pedal is released ? Sometime a vacuum bleed isn't as good as the old fashioned "push fluid through" technique. And BTW - if you had good flow at each of the calipers while bleeding - it's not real likely that either a brake line of the ABS unit is clogged.
Start the car with your foot lightly on the brake pedal - the pedal should move down a bit when the engine lights up, and generates vacuum.
Try disconnecting (and plugging) the vacuum line to the power brake booster - the power assist should be gone after a couple of brake applications. Does the braking with no power assist get much worse, or is it comparable ??? Obviously - it should be quite significantly worse.
BTW - Have you tried the old 2 person bleed technique, where one person depresses the brake pedal while the other opens the bleed valve, and closes the bleeder before the brake pedal is released ? Sometime a vacuum bleed isn't as good as the old fashioned "push fluid through" technique. And BTW - if you had good flow at each of the calipers while bleeding - it's not real likely that either a brake line of the ABS unit is clogged.
Thread Starter
Instructor
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From: New Jersey
Normally - the idea is to get the old fluid and air out of the longest run of brake line first - particularly before ABS cars - where the rear brakes were fed from one common line that was "Tee'd by the diff. However in the C4 - the ABS unit is in the back - so the longest run of brake line is to a front caliper.
IMHO - it really doesn't matter which wheel you bleed first in an ABS car. Just flush fluid thru until you get nearly clear fluid from each caliper.
IMHO - it really doesn't matter which wheel you bleed first in an ABS car. Just flush fluid thru until you get nearly clear fluid from each caliper.
Thread Starter
Instructor
Joined: Jul 2020
Posts: 227
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From: New Jersey
Team Owner
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From: Outside the Quick Stop N.J.
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Instructor
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From: New Jersey
As an aside: I've seen some forum threads where people suggest you MUST measure the runout and shim brand new rotors on these cars. That seems a bit nuts to me, but assuming there's a runout issue, I would expect that to show up as brake pedal pulsation, not just overall lack of braking performance, right?
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Thread Starter
Instructor
Joined: Jul 2020
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From: New Jersey
Thanks, that's kind. Do you have access to a Tech1? If so, where are you located? My parents live in Central NJ and there's a chance I could limp the car there with weak brakes if it gave me some further troubleshooting options.
Safety Car
Joined: Mar 2017
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From: LeClaire Iowa
Oil Producer
Joined: Apr 2014
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Thanks for the pic. Once I sort out whatever's going on with the wheel speed sensor, I'll take the car out and do this and see if we get any different results when bleeding.
As an aside: I've seen some forum threads where people suggest you MUST measure the runout and shim brand new rotors on these cars. That seems a bit nuts to me, but assuming there's a runout issue, I would expect that to show up as brake pedal pulsation, not just overall lack of braking performance, right?
As an aside: I've seen some forum threads where people suggest you MUST measure the runout and shim brand new rotors on these cars. That seems a bit nuts to me, but assuming there's a runout issue, I would expect that to show up as brake pedal pulsation, not just overall lack of braking performance, right?
never ever heard of that nor done that.
i mean i did find the brake system to be severely lacking in roaches ive bought but the solution was to simply get all the components working.
check your wheel bearings for movement
Team Owner
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From: Outside the Quick Stop N.J.
I would be happy to help you out.
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Instructor
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From: New Jersey
That's a truly generous offer! I grew up near Toms River - so not too far from there. I'm going to continue battling with it here on home turf for another few rounds, but if the offer still stands when I throw in the towel, I might well reach out and take you up on it!
Team Owner
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From: Outside the Quick Stop N.J.
Thread Starter
Instructor
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From: New Jersey
Thanks for the pic. Once I sort out whatever's going on with the wheel speed sensor, I'll take the car out and do this and see if we get any different results when bleeding.
As an aside: I've seen some forum threads where people suggest you MUST measure the runout and shim brand new rotors on these cars. That seems a bit nuts to me, but assuming there's a runout issue, I would expect that to show up as brake pedal pulsation, not just overall lack of braking performance, right?
As an aside: I've seen some forum threads where people suggest you MUST measure the runout and shim brand new rotors on these cars. That seems a bit nuts to me, but assuming there's a runout issue, I would expect that to show up as brake pedal pulsation, not just overall lack of braking performance, right?
OK. We managed to wrangle out the offending wheel speed sensor today, cleaned it off and reinstalled it. That DID clear the ABS code. Had the boy take it to a gravel parking lot and perform a half-dozen ABS activations. It did activate and modulate the brake pressure, etc. No discernible difference in braking performance. He's going to bleed it again and see if anything changes, but let me ask another question about ABS module failure modes.
Is it possible that the seals in the solenoids are failing and allowing fluid to leak past the intended circuit (to the brake pads) and instead into the accumulator even when the system isn't activating? This would explain why no braking seems to occur until we've already gone through most of the pedal travel.
Assuming we get no improvement via bleeding (which I'm not expecting at this point), our next step is probably going to be bypassing the ABS altogether by joining the pipes in the rear hatch to return to "old school" style brakes. If that fixes things, then I guess my explanation above might hold water?
Either that or I'm going to set the car on fire and forget about it... tune in next week to see which way this goes...
Advanced
Joined: Jul 2017
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From: Texas
Just a thought if it hasn't been done yet, lift a front wheel off the ground and spin it. Does it spin freely? If it spins freely, about how much brake pedal travel does it take to stop the wheel while spinning? Maybe the booster rod needs adjusting.
Another one that's unlikely, chalk the tires at each valve stem and make sure they don't rotate on the wheel.
Edit:
In the interest of speeding the bleeding/actuating ABS along, you should be able to actuate it with all 4 wheels off the ground. Start it, turn off ASR, put it in drive and get the rear wheels moving then hit the brakes. I did this on my 93 before I got a scan tool.
Another one that's unlikely, chalk the tires at each valve stem and make sure they don't rotate on the wheel.
Edit:
In the interest of speeding the bleeding/actuating ABS along, you should be able to actuate it with all 4 wheels off the ground. Start it, turn off ASR, put it in drive and get the rear wheels moving then hit the brakes. I did this on my 93 before I got a scan tool.
Last edited by Eilias; Apr 9, 2022 at 06:38 PM.
Thread Starter
Instructor
Joined: Jul 2020
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From: New Jersey
Just a thought if it hasn't been done yet, lift a front wheel off the ground and spin it. Does it spin freely? If it spins freely, about how much brake pedal travel does it take to stop the wheel while spinning? Maybe the booster rod needs adjusting.
Another one that's unlikely, chalk the tires at each valve stem and make sure they don't rotate on the wheel.
Edit:
In the interest of speeding the bleeding/actuating ABS along, you should be able to actuate it with all 4 wheels off the ground. Start it, turn off ASR, put it in drive and get the rear wheels moving then hit the brakes. I did this on my 93 before I got a scan tool.
Another one that's unlikely, chalk the tires at each valve stem and make sure they don't rotate on the wheel.
Edit:
In the interest of speeding the bleeding/actuating ABS along, you should be able to actuate it with all 4 wheels off the ground. Start it, turn off ASR, put it in drive and get the rear wheels moving then hit the brakes. I did this on my 93 before I got a scan tool.
https://www.zip-corvette.com/68-76-p...ment-tool.html
I can't imagine that the tires are spinning on the wheels (especially since we just had one tire swapped to a new(used) wheel to fix a rim leak. But I guess at this point, I'm going down rather lower-probability troubleshooting paths, so anything is worth checking.
Oil Producer
Joined: Apr 2014
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Thanks for the ideas. Been down these roads a bit too. The major issue is actually that it takes about 3/4 of the brake travel before any real braking action occurs. I actually bought this brake booster pin measurement/adjusting tool and verified proper booster pin adjustment:
https://www.zip-corvette.com/68-76-p...ment-tool.html
I can't imagine that the tires are spinning on the wheels (especially since we just had one tire swapped to a new(used) wheel to fix a rim leak. But I guess at this point, I'm going down rather lower-probability troubleshooting paths, so anything is worth checking.
https://www.zip-corvette.com/68-76-p...ment-tool.html
I can't imagine that the tires are spinning on the wheels (especially since we just had one tire swapped to a new(used) wheel to fix a rim leak. But I guess at this point, I'm going down rather lower-probability troubleshooting paths, so anything is worth checking.
im sure you covered this upthread but have you bench bled the master cyl?
Thread Starter
Instructor
Joined: Jul 2020
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From: New Jersey
To be fair, there was A LOT of crud in the original fluid reservoirs, so I don't find it impossible that it trashed some seals internal to the ABS somewhere. For example, here's a pic in the middle of it where I had cleaned out one of the reservoirs and you can see the stark contrast with the condition of the other:
