I have come to truly hate the braking system on my son's 1990 base coupe.

A few months ago while driving the car, we noticed some clunking/rattling from the front end. It turned out the anti-rattle springs on the front pads were shot. When trying to replace them, the "spring clips" that slot into the calipers broke, so we ended up putting on new pads and rotors. I just popped open the master clyinder reservoir when compressing the calipers, not the brade bleeder, so shouldn't have introduced any air. Ever since then, I've noticed a really soft brake pedal and exceedingly poor braking performance (pedal had to basically go all the way to the floor for you to get even moderate braking and there was no possibility of locking up the wheels and/or activating the ABS).

Thought maybe the back pads/rotors were worn or suspect and it's an old car anyhow, so I replaced those. Brake booster was cracked and had a leak, so I replaced that.

Bled the system about 5 times. Took off the master cylinder, cleaned, bench-bled and reinstalled it. Still no luck. Figuring maybe some seals were shot in the MC, we replaced that this week. It's better, but STILL a very soft pedal. Going to try bedding in the pads in case they didn't bed well with the low braking force, but I'm running out of ideas here.

Anyone have any idea what could be going on? I don't see any blisters or visible expansion in the brake hoses, but I'm thinking about changing them out to stainless since I'm just running out of remaining brake system components to suspect. There are no leaks anywhere, so it seems hard lines and calipers are good. Is there anything in the functionality of the 1990 ABS module that could trap air that wouldn't be removed just doing a "normal" bleed? Would appreciate any additional advice since I'm pulling my hair out a bit.

 

My brakes were NOT doing that but I went to SS braided lines and it did firm up a little. You could have a brake line problem (internally) so replacing these if original is prudent anyway... With all of the problems you had you may have gotten air trapped in the ABS module I guess? I have no experience with this as I've never had a problem. I also used a vacuum bleeder at each wheel.

Do you have the FSM for your year? Mine is a 93 and it talks about bleeding at the "modulator valve prime pipe if air has been introduced through low fluid level or by disconnecting brake pipes at the master cylinder". For mine, the bleed screw would be on the ABS line junction/header thing behind the driver's seat in the compartment.

 

I had a really hard time with my previous 89 convertible and those brakes. When I got it the ABS/Brake light was on. The rear brakes would not bleed a drop of fluid when going through the bleeding process. I tried several different vacuum pumps and methods for bleeding. Replaced the power booster, replaced the master cyl, replaced the calipers, brake lines, etc. Finally I replaced the ABS pump and that cured it. I found a thread on here that someone opened one of these ABS pumps up and they're easy to clean. The ABS pump the guy cleaned out had dirt and gunk trapped in the ports. So you may want to look into that. It didn't cost anything to open it up and clean. I still have my original and have it on the to do list to open it up just to see.

 

Thanks for the info. I may end up going to braided stainless line just so that I don't have him driving around with original rubber hoses on a 30 year old vehicle. I do have the FSM and for this model year it doesn't mention anything unusual regarding the bleed procedure related to the ABS module. I know that the method of control changed in the later years and there may have been separate pumps in the ABS system that therefore led to them having to be actuated in order to bleed, but I was under the impression that wasn't an issue with the 1990.

 

That's interesting to note. I think the 89 and 90 ABS modules were roughly the same. Mine doesn't show the ABS/Brake light and I can hear the unit go through its self-test without issue when I hit 6MPH. We also ran a whole bunch of fluid through all four wheels and purged them all until new clean fluid was coming out. I would also have expected that if a blockage was the issue the problem would be too firm a pedal without braking force. Too mushy a pedal suggests to me that fluid is displacing somewhere - just not in a way that generates good braking force.

 

I suggest activating the abs while bleeding via a tech1,2 scanner or other make scanner that has that ability. A lot of air gets trapped in the abs due to its location.

 

 

Thanks, that is a fantastic video resource! Now let me ask another question: Other than shelling out like $800 plus for an eBay Tech1 and the appropriate cartridges, does anyone else know the most readily available method for actuating the ABS? Would a Snap-On MT2500 do it? I have a Moates ALDL cable, does anyone know if there's a utility to exercise the ABS module similarly?

 

My Solus will do an automated bleed, so I think a MT2500 should as well.

 

from what I have herd a mt2500 will do it, and I do not believe there is any app or file for tuner pro that will activate it,
I have activated th abs in the past with out a scanner but do not recommend the method because there is a
chance you could fry something.

 

Do you by chance know what braided stainless lines you went with? Or for that matter does anyone have specific experience with these:

https://www.cssbinc.com/1988-93corve...kehoseset.aspx

I picked up a set of them through Zip Corvette and I'm planning to install them but noticed a key difference between these and the standard hoses. It looks like the original rubber hoses had a terminating banjo fitting shaped like a large "block."



The new ones have a much smaller round banjo fitting.



I know in theory they will still seal up the same, but it seems like the big block would be a bit more resistant to loosening over time since nothing could torque them and force them to rotate. Anyone have any thoughts or experience with the CSSB hoses?

 

If tightened up properly the SS hoses won't rotate either. These hoses don't have much give or twist in them, so make sure you mount them properly on the caliper before screwing them on the brake lines. There is little room for error, but they aren't hard to install either.
Besides missing hardware (purchased at a discount from a local to me vendor) I have had no issues with the CSSB hoses.

 

Are you getting strong flow when you crack the bleeder?

 

Yes, we flushed out all the old dark-looking fluid so could see easily that we were getting good flow at all four corners. When I change the hoses out (probably today) I'm also installing speed bleeders since it appears my fate is to bleed the brakes on this car approximately 100 times. I bought a motive power bleeder as well, but cannot for the life of me get it to seal up correctly on the MC reservoir and hold pressure so haven't been able to use that yet.

If replacing the hoses doesn't finally resolve it, then my next theory will be that there's air trapped in the ABS accumulator that's getting pumped back into the line every self-test cycle after a bleed causing us to have to start over again. I'll have to buy some sort of scan tool capable of actuating the module as in the video above so I can finally purge that all out during a bleed.

 

i dont have a tech1 or anything like it. the way i handle soft/poor brakes on a c4 is to completely flush, then go to a small gravel patch and do about ten rolling abs activations. then re-bleed. i use the master cyl and brake pedal with clear pcv hose into the 2litre coke bottle trick, saw chris fixs video. and my iphone to video the fluid as it comes out. i can see air and dirty fluid this way. has worked every time. and after i get the abs pump working like that it always kicks out bubbles and sometimes a darker tinge of fluid.

i justify two bleeds in a row on a new to me c4 because everytime i give the abs pump a good solid workout i get more crud out. so worth it.

note i never get up to big speed while activating the abs. i dont want rocks anywhere. rolling idle all you need.

for street only i use the cheapest dot 3 i can find. for track cars i use motul 660 and so far been using powerstop “track day” kits with j55’s. mucho brake dust with powerstops track day kits. i run car through the wand wash after the track to scrub it off before it adheres into the rims!!

So far worked great for an 86e, a 91ZR1, and a 99mn6. i did get started on a shoe string budget.

 

^^^ Works ^^^

 

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!

 

Thanks for the advice. I'm willing to give that a shot. At this point I'm at bleed number 12 on this thing or something, so certainly not scared of bleeding again.

Today we went out and changed out the hoses to new braided stainless units. I also installed speed bleeders and then bled the system again. When my son was backing it out to see if it was braking better (it wasn't), the SERVICE ABS light came on. No idea how anything we did today would have caused that, but it appears I'll be trying to read those codes tomorrow and perhaps whatever fault is now showing will give me some clue what's going on. But I guess with a steady ABS light on, there's no way for me to use your method since the ABS will be disabled.

So the story continues...

 

I pulled the offending ABS code today by grounding the ALDL pin H (still don't have a scanner capable of working with the ABS). It showed code 31 - RR wheel speed sensor fault. I'll pop the wheel off and take a look to see if I somehow damaged or disturbed something while changing out to the stainless steel hoses. Nonetheless, I would say this code means the ABS unit is NOT the cause of the poor braking performance. One question I do have for diagnosis experts: Is it possible for the ABS to show code 31 AND one of the codes marked with a "*" in the FSM that it indicates can only be read with a scan tool (such as solenoid valve faults)?

Where would you guys suggest I look next? I have literally replaced every component in the system aside from the calipers and ABS module. And since the calipers aren't leaking, I can't imagine how they would cause a soft pedal.

Is there any chance that some sort of seal has gone bad internal to the solenoid and even when ABS isn't active is allowing fluid to "leak" across the boundary to the accumulator?

 

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.