Soft Brake - Scratching my head
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
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From: TX
Soft Brake - Scratching my head
1996 Base model C4. The car sat for many years before I bought it. I've been working on cars, trucks, and motorcycles for 50 years but I can always learn something and accept help.
Replace MC (Bench bled), rebuilt front and rear calipers (bled at every wheel), rotors have been turned, and new pads all the way around. ADD - also replaced all 4 brake hoses.The vacuum booster appear to be working as there is a drastic change when the vacuum line is disconnected (Yea it runs bad when it's off). No leaks visible anywhere. Haven't touched the proportioning valve, thinking that only varies the pressure front to back.
Problem is, the brake pedal feels like mush and requires too much travel before beginning to brake. The car has ABS, but I wouldn't think that is an issue until it senses a wheel locked (NO ABS alarms or codes). All testing has been in my driveway, it will stop, but not as it should and without much pedal movement.
Please, what am I missing or should I check. I definitely wouldn't be comfortable driving it as is.
Replace MC (Bench bled), rebuilt front and rear calipers (bled at every wheel), rotors have been turned, and new pads all the way around. ADD - also replaced all 4 brake hoses.The vacuum booster appear to be working as there is a drastic change when the vacuum line is disconnected (Yea it runs bad when it's off). No leaks visible anywhere. Haven't touched the proportioning valve, thinking that only varies the pressure front to back.
Problem is, the brake pedal feels like mush and requires too much travel before beginning to brake. The car has ABS, but I wouldn't think that is an issue until it senses a wheel locked (NO ABS alarms or codes). All testing has been in my driveway, it will stop, but not as it should and without much pedal movement.
Please, what am I missing or should I check. I definitely wouldn't be comfortable driving it as is.
Last edited by TSheff; Mar 30, 2024 at 01:37 PM. Reason: add-on
Drifting
Joined: Aug 2009
Posts: 1,971
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I do believe the ABS modules in the bin behind the seat changed over the years, but my 94 has a bleeder on the module in the bin. Consider bleeding there as well if the 96 too has a bleeder at that location.
also, soft lines to the calipers?
also, soft lines to the calipers?
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
Likes: 47
From: TX
Headed out right now to check for a bleeder on the ABS.
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
Likes: 47
From: TX
The Motive Pressure Bleeder should be here Thursday and I'l post results after I've had a chance to use it.
Thank You
Instructor
Joined: May 2023
Posts: 120
Likes: 10
1996 Base model C4. The car sat for many years before I bought it. I've been working on cars, trucks, and motorcycles for 50 years but I can always learn something and accept help.
Replace MC (Bench bled), rebuilt front and rear calipers (bled at every wheel), rotors have been turned, and new pads all the way around. ADD - also replaced all 4 brake hoses.The vacuum booster appear to be working as there is a drastic change when the vacuum line is disconnected (Yea it runs bad when it's off). No leaks visible anywhere. Haven't touched the proportioning valve, thinking that only varies the pressure front to back.
Problem is, the brake pedal feels like mush and requires too much travel before beginning to brake. The car has ABS, but I wouldn't think that is an issue until it senses a wheel locked (NO ABS alarms or codes). All testing has been in my driveway, it will stop, but not as it should and without much pedal movement.
Please, what am I missing or should I check. I definitely wouldn't be comfortable driving it as is.
Replace MC (Bench bled), rebuilt front and rear calipers (bled at every wheel), rotors have been turned, and new pads all the way around. ADD - also replaced all 4 brake hoses.The vacuum booster appear to be working as there is a drastic change when the vacuum line is disconnected (Yea it runs bad when it's off). No leaks visible anywhere. Haven't touched the proportioning valve, thinking that only varies the pressure front to back.
Problem is, the brake pedal feels like mush and requires too much travel before beginning to brake. The car has ABS, but I wouldn't think that is an issue until it senses a wheel locked (NO ABS alarms or codes). All testing has been in my driveway, it will stop, but not as it should and without much pedal movement.
Please, what am I missing or should I check. I definitely wouldn't be comfortable driving it as is.
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
Likes: 47
From: TX
Not going to say it's not possible, I'll say not probable, third one - Original one - New one - replacement New one after the first was warrantied. It's more probable the first New one was not bad. I kept the original as a spare and may rebuild it for another day.
Instructor
Joined: Jan 2013
Posts: 128
Likes: 26
Soft brakes are a common topic on here (too). I rebuilt the entire system and had a really hard time bleeding the system. Required lots of fluid (about 2 quarts if I remember correctly) both gravity and pedal bleed and finally a few hard brake applications to get ABS to activate, followed by a manual bleed "just to be safe".
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
Likes: 47
From: TX
Soft brakes are a common topic on here (too). I rebuilt the entire system and had a really hard time bleeding the system. Required lots of fluid (about 2 quarts if I remember correctly) both gravity and pedal bleed and finally a few hard brake applications to get ABS to activate, followed by a manual bleed "just to be safe".
If that doesn't work I'll definitely eliminate the ABS totally.
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Brett Foote
Instructor
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From: South Texas
I had the same problem on my '96. It took three master cylinders, an abs module, and a trip to GM. First I got a Raybestos MC (new), failed in a few days. The pedal was impossible to push down. Then I went with Doorman, also new. First one started leaking from the pressure switch, so they sent me another one. Which has been great, so far. I also did the same as you, except I flushed the system, prior to the install. Don't want any nasty fluid going through a clean caliper lol. Anyhow, brake still felt spongy after a pressure bleed. Turned out to be the ABS pump, ie. seals were bad. Once I got a replacement, the brakes felt great, not perfect, just great. So I found out that the valves inside the ABS has to be purged. Which you can do with a Tech2/MDI2, set off your ABS on gravel roads, or take it to GM. Once that was done, now we can say it's perfect!
Burning Brakes
Joined: Sep 1999
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Cruise-In II Veteran
Something that many are not aware of is that the old standard brake bleeding/flush sequence (RR-LR-RF-LF) doesn't apply to the C4. The correct sequence is: RF-RR-LR-LF. (Corvette Service Manual sections 5-7 and 5-8).
The reason for this is the location of the ABS module requires a different layout of the brake tubing than the old style. i.e. the longest tubing run is now to the RF as opposed to the old style (non-ABS0 which has the RR as the longest run. Brake bleeding should be longest first then work down to the shortest).
Doing the old way has a good chance (even with pressure bleeding) of leaving air in the system.
The reason for this is the location of the ABS module requires a different layout of the brake tubing than the old style. i.e. the longest tubing run is now to the RF as opposed to the old style (non-ABS0 which has the RR as the longest run. Brake bleeding should be longest first then work down to the shortest).
Doing the old way has a good chance (even with pressure bleeding) of leaving air in the system.
Last edited by Wildride; Apr 3, 2024 at 12:12 PM.
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
Likes: 47
From: TX
Something that many are not aware of is that the old standard brake bleeding/flush sequence (RR-LR-RF-LF) doesn't apply to the C4. The correct sequence is: RF-RR-LR-LF. (Corvette Service Manual sections 5-7 and 5-8).
The reason for this is the location of the ABS module requires a different layout of the brake tubing than the old style. i.e. the longest tubing run is now to the RF as opposed to the old style (non-ABS0 which has the RR as the longest run. Brake bleeding should be longest first then work down to the shortest).
Doing the old way has a good chance (even with pressure bleeding) of leaving air in the system.
The reason for this is the location of the ABS module requires a different layout of the brake tubing than the old style. i.e. the longest tubing run is now to the RF as opposed to the old style (non-ABS0 which has the RR as the longest run. Brake bleeding should be longest first then work down to the shortest).
Doing the old way has a good chance (even with pressure bleeding) of leaving air in the system.
Thread Starter
Instructor
Joined: Aug 2010
Posts: 119
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From: TX
Did a power bleed, the did some driveway test. It cycled the ABS a few times, but it is still not what I would call acceptable.
I may repeat the process again today. Getting very frustrated.
I may repeat the process again today. Getting very frustrated.
Melting Slicks
Joined: May 2019
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2025 C4 of the Year Finalist - Unmodified
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Working the ABS on a gravel road and then bleeding again afterward solved that problem for me.
Thread Starter
Instructor
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From: TX
I'll scout the area and see if I can find a gravel or shell road nearby. Don't want to wander too far with the brakes as they are. I also need to get a can of 5 gallons of fuel to add to the tank before going. Don't want to chance running out and the nearest station is just far enough away to do that.
Forgot to mention the bonus "Big" female Black Widow I found living behing the LR rotor. Her abdomen was about 3/8". Sadly she died.
Weather is going to suck for a few days and we are keeping grandchildren this week, so it will be a week before I do any further work or testing and if the weekend clears up I really need to get out for some riding time on my Harley.
Forgot to mention the bonus "Big" female Black Widow I found living behing the LR rotor. Her abdomen was about 3/8". Sadly she died.
Weather is going to suck for a few days and we are keeping grandchildren this week, so it will be a week before I do any further work or testing and if the weekend clears up I really need to get out for some riding time on my Harley.
Melting Slicks
Joined: May 2019
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From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
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To heck with a bunch of spiders. Especially black widows? Pft. I'd spray the entire car, every single inch. Sheeeit. Spiders can kiss mah ***...
Racer
Joined: May 2007
Posts: 306
Likes: 49
From: Florida
Im in the same boat, as they do not make the ABS pump anymore, once those seals go the car is worthless i suppose...After doing everything you did to my 96, i confirmed my issue is in the rear circuit and is NOT the calipers or the rubber lines, becuse i leliminted them by crimping off both rubber lines, and capping off the front circuit at the M/C and still get a sinking pedal. Block off the rear circuit enrirely at the M/C and perfect pedal, so NOT M/C or front circuit in anyway. So I finally Took it to a shop that specializes in General Motors vehicles and was told the ABS pump/module needs replaced. They do not make reproductions or rebuild kits he says and I can't find any information on how to rebuild one. So the mechanic said well you're just gonna have to live with a soft brake pedal in the rear. Well I sure as hell hope whatever issue that has happened inside my ABS pump doesn't happen to the front circuit because then I would be totally ******. So I guess when you have these old C4s and your ABS pump starts leaking you just have to get rid of the car i suppose, throw them away especially if both circuits are weeping in the ABS pump/module...well done GM well ******* done.
Safety Car
Joined: Mar 2017
Posts: 4,195
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From: LeClaire Iowa
You have lost me with the negativity. Just have it rebuilt. Not a giant big deal. Dan EDIT: The rebuild cost is way way less than GM would be charging you if, in fact, they did have a new one to sell you.
MTechnique ABS (mtechniqueauto.com)
MTechnique ABS (mtechniqueauto.com)
Last edited by Whaleman; Apr 30, 2024 at 06:16 PM.
Safety Car
Joined: Mar 2017
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From: LeClaire Iowa
Why is it a stretch to believe that a complicated piece needs to be cleaned and rebuilt in an almost 30 year old car? GM has no control on how often the owners completely flush the brake fluid. In your opinion was GM charged with designing an ABS and brake system than can operate perfectly on 30 year old DOT 4 brake fluid? I don't think so. Dan
Thread Starter
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From: TX
Have mine in the shop as I type. They are not rebuilding the ABS but the have been tasked with completely flushing and bleeding the system. I verified that they have the programmer to operate the ABS while doing the flush. I'll see what the results are. Thanks