Another thing to check is the line pressure at each caliper. I had an issue with the 4WDB conversion on my 68 Caprice, I used a brake line pressure gauge inserted into the bleeder port at each caliper to pinpoint the problem.

 

I know this thread is over a year old, but....just wondering if your issue was resolved, as I am having some similar issues diagnosing my '88.

David

 

Unfortunately, we only ever got the brakes "good enough" that I wouldn't lose sleep knowing my son was driving it. He STILL can't lock the wheels up on dry asphalt - which doesn't seem right. I have some additional things in the wings to possibly try: Another used ABS unit I bought that I might try to take apart and rebuild; a flaring kit and some fittings in case I decide to just remove the ABS altogether, and a new power bleeder adapter that maybe could force out any remaining air bubble if one is still hanging out in the ABS unit.

I hope you manage to get your issue sorted and I'd be happy to discuss everything I've tried so far, but don't want to set myself up as an expert since I never did get it fully sorted to my liking. Good luck.

 

unless you have a toasted brake booster, multiple abs activations, bleeds to remove air (i still use the 1 man bleed into a coke bottle with hole drilled in cap and hose under bottles fluid level, will solve your issue.

ive now done this on 3 additional c4’s (89 and 2 90’s) to what was mentioned above in 2022.

My 89 took several abs activations per day over a week. Now, they are the best brakes i have. I started a thread to it actually phaze, nobody can really explain why it works. dont give up. your boys brakes should be phenomenal unless u have a mechanical component failure. cheers, VT.

 

No mention of what Vendor supplied the replacement parts so there for one has to assume that a chain of auto parts stores was used. If you jump over to the C3 forum and look at brake problems some people have and some don't the one thing in common is the use of Corvette Vendors parts vs Auto Parts parts stores. People have shown pictures of C3 Calipers from Auto Parts chains with the Caliper Bore painted before the Pistons were installed. Since You replaced the Vacuum Booster with a metal one could possibly mean its some sort of generic that might fit other cars. Do you feel any play in the brake peddle ? In other words do you think the peddle is using up some of its travel before the master cylinder moves? Do you still have the OEM cracked one ? With no Vacuum boost at all it will still be able to lock up the wheels you will just need to use a lot more peddle pressure to do it.

 

It was a bit of a hodgepodge of vendors over the course of several months. Generally, I try to go to the local independent auto parts store near me whenever I can. From them, we purchase the pads and rotors - all ACDelco Gold. I also bought the metal brake booster from there - it was a Cardone generic replacement since the plastic ones weren't readily available anywhere. Braided stainless lines were from Corvette Stainless Steel Brakes (via Zip Corvette if I remember). The master cylinder was definitely from Zip Corvette, but I would note that the behavior with the new master wasn't much different than with the OEM one that we pulled out. As for play in the pedal, no there's no slop. I also ended up buying a pin measuring/adjustment tool to make sure the pin of the booster was set perfectly for engaging the master cylinder. I really truly think the only items left we could be struggling with are air trapped in the ABS module or a seal failure internal to the ABS module causing these symptoms (though I contacted the rebuild service noted above and he said that although it's possible for this sort of failure mode, it's not the most common one). I suppose it could possible by a caliper issue as well, but they all seemed free and none of them were leaking, so I'm not keen on dropping money on new calipers or rebuilding them when they don't exhibit any of the normal failure symptoms.

 

Working on my 1990 I did everything wrong. As in I needed to replace a rear brake line and all the hoses and I let everything run dry and they say you should never let an ABS run dry. That being said when I Bleed my system all was fine the first time out. I built my own Vacuum Bleeder I had one of the Mighty Vacs way back when but its not enough. I use a glass milk bottle for my catch can and put it under a Vacuum using an AC Vacuum Pump. The Vacuum off the Intake would do the same thing. Then I bleed each Caliper. I have never Bench Bleed a Master no need. Once I have Fluid at all 4 corners I reverse and pressure bleed using the Peddle. This is most likely the end of the day and let it sit and see if a bubble or two makes they way the next morning.
If you had a bad caliper then the car should pull hard one direction or another. If the fronts or rears lock and not the other then its not one system but both. So this is leading us back to the Master. By removing a Line and plugging it You should be able to mash the peddle as hard as you can and not budge it I shouldn't matter which one or both. You could also Block all four lines at the Calipers and it should do the same thing. If you can't move the master by itself, but you can when all the Calipers lines are blocked then you have a problem in between. And it really does't matter if the ABS unit is good or bad, unless its leaking Brake Fluid you should have a rock hard peddle. If not you are compressing Air. I know you have bleed the system a bunch of times, but I would start there using another method. Next time you drive the car do it in slippery conditions and feel if the Peddle ABS chatter when you mash it.

 

My 91 had no leaks, passed local (tough) inspections etc. but felt almost impossible to stop, changing pads made no difference. As part of suspension rebuild I took apart the calipers. The dust boots were torn and 2 out of 4 pistons were very hard to remove, I had to destroy one to get it out. I replaced the pistons, polished the bores etc complete rebuild. Parts were cheap and now the brakes are a dream. Excellent feedback and control.

Bleeding took multiple times and required ABS activations.

 

What did you use to polish the bores? Since seal kits are $5. I think I'll take my front calipers apart and replace the seals while they are off for new pads and hoses. I assume with the old seals out that you can clean it with brake cleaner?

And what lube did you use for the internal seal? I see people recommend using brake assembly fluid as they say the piston is easier to push back in and some say you could get a leak if you use brake fluid for lube.

 

For the better bores I just used a cotton cloth and metal polishing compound hand/finger applied. The rougher bores I polished with different sized Dremel polishing tools, again with metal polishing compound (gently..). First washes with degreaser+soapy water, final clean with liberal use of brake cleaner.

I used a light coat of 'professional' brake lube for all rubber parts, it was either Permatex or CRC. You must use something on the piston seals otherwise there's a good chance they tear or twist. Assembly was super easy especially compared to disassembly.

 

this may be helpful:
https://www.corvetteforum.com/forums...post1606445369

I used some fine wet sand paper if something needed to be touched up/deburred, and replaced one piston

 

I think I already have some metal polishing compound. I do have a package of small foam polishing discs, the kind that you can put in a drill. Probably would use that to do a quick polish of the bores.

 

As an aside to everything else mentioned here, I'm prettty sure the ECM bracket on the '90 sits between the booster and the firewall, thus rendering rod adjustment mandatory before even thinking about bleeding anything.