I have a 1979 C3. Ever since I purchased this car, the brake light comes on and off and pulls when braking hard. The brake light always turns off after pumping the brakes one or two times. When I hit the brakes hard, it usually pulls to the left, however, it also pulls to the right occasionally. Recently, the front right caliper was dumping brake fluid and I thought I had finally identified the issue, so I replaced that with an AC Delco caliper, which was also dumping fluid straight out of the box. Got another AC Delco caliper rebuilt by a different company, and within two days, it is pulling again. This caliper is not visually leaking.

It seems that when I bleed the brakes, everything works fine for a little bit. But within a couple of drives, the problem comes back. The fact that the direction in which the car pulls changes sometimes when braking leads me to believe that the issue is the proportioning valve (I believe the shop manual calls it a combination valve). But from what I've read on other forum posts, most people seem to say that that valve is rarely the actual issue.

What next steps would you take towards diagnosing this issue?

EDIT: The braking problem seems to be entirely solved now. I ended up replacing the master cylinder, but I'm not sure that was actually the fix. When bleeding the rear brakes after replacing the master cylinder, I noticed for the first time that those calipers have two bleeders on the top. I don't particularly remember if I had noticed both before and bled both, but I'm pretty sure I didn't. A whole lot of air came out of the rear left caliper. Hopefully the reason the air got in there was due to the master cylinder replacement. If not, then perhaps the problem isn't solved yet. Thanks for the suggestions, if the problem comes back I'll update here again.

 

For me it was the master cylinder, car would always pull to the right. And the master cylinder I had on there was not that old, I chased my tail on that one . Good luck

 

Another thing that can cause your car to pull both left and right could be loose shims in the trailing arm pocket or the front TA bushing may be worn out.

 

Have ALL rubber brake lines (one at each wheel and one in center-rear) been changed? Those may look great on the outside; but inside they are "toast" when they get old. PITA to change them because of soft steel hex nuts. But, that must be done to eliminate brake system issues and have a SAFE vehicle. Once those are changed out, verify that the valve in the brake distribution block is in its normal "centered" position. Then, bleed master cylinder (still assembled to car) and all calipers (both bleeders in rear calipers!). That should fix your issues.

 

On front brake systems.
A pull the left in your case means the right side is doing nothing.
Then it starts working again. Why?
Most likely rubber hoses. Internals have collapsed.

By the way, both front hoses and ONLY the front hoses get a copper washer between hose fitting & caliper.
Not just any copper washer, but a soft metal known as "crush washer".
That will stop most common leaks at that junction.

Better sellers of front calipers will include the special washer in the box.

 

I just went through this headache and the culprit was the hoses. I rebuilt both front calipers and converted them to O-ring seals and did a full bleed of the front lines and the car still pulled to both the left and right. With light pressure on the brakes, the car would pull left. Applying more pedal pressure the car would then pull to the right. Bit frustrating but it worked out.
I replaced both front hoses and that solved the problem. Currently working on the rear hoses and calipers since Im already here.

 

If you didn't recently replace those brake hoses yourself, the best way to test them is to get 4 golf tees, and also some vise grips. Cut the hoses in the center, and plug the master cylinder side with the golf tees. If you can't jam a golf tee in the hose, it has swelled internally, and it's a good thing you checked. Use the vise grips (or a special brake hose clamp) to keep the master cylinder from draining while you make repairs.

I'd highly recommend stainless braid brake hoses. Just make sure they are PTFE on the inside, like Goodridge makes, or Russell Performance.

https://www.partsgeek.com/5kmpv4d-ch...-hose-kit.html

What do your trailing arm and strut rod bushings look like?

 

Long ago, I put Corvette America stainless steel sleeved calipers on my '68. They worked great for a long time, but then they began leaking. I changed seals and they were good about a year before leaking, pulling, and so on. I changed seals again, changed hoses, and all was good just under a year. I tried to get Corvette America to take them back, redo them, or exchange them under their Lifetime Warranty, but they refused. So I took the advice of a friend and picked up the ones from Advance Auto. They are as good as everyone else, AND they have a warranty that they never argue with you about. That has been eight years ago and I have had no issues with them. I replaced all of them and the brake hoses back then.
it might cost a bit more to do everything, but you know they work and you won't worry when that car pulls in front of you if you are going to stop or not. That peace of mind is worth more than most know.

 

Thanks for the shortcut to braided hoses; sadly they are out of stock but will keep and eye out for them. Good price btw

 

https://www.summitracing.com/parts/RUS-692000

$115 at Summit, in stock.... that's pricey. I got em for $75 a few months ago. Definitely worth the upgrade over rubber. I love em.

 

They're still rubber inside. The braided jacket looks nice; but expensive, just for "looks". Just my $.02

 

This is false information. The stainless steel braided brake hoses made by Russell Performance, by Goodridge, and likely a few others, are PTFE on the inside (Teflon). They really do make a noticeable difference in braking (much more noticeable in a car I had with manual brakes, as others have noted).

It is not a "for looks" item. It is an actual performance item, used in real race cars, and unlike many race car solutions, actually last longer than the stock parts. If you order a set from a reseller, drill down on who made them, and make sure they really are PTFE lined.

 

Goodridge lines are definately PTFE lined

 

Proportioning valve is not likely a problem. It's about front to back not left to right. You did say you had been losing brake fluid on one side. Was that the left front? If so..... I know you said you replaced that caliper. Did you also replace pads on both sides? Were you very careful to be sure you cleaned up the disc and any brake fluid remaining anywhere and everywhere that may have and/or could be contaminating the pads on the previously leaking side? I only ask because it takes such a very small amount of contamination to cause a "pull" to the side than isn't contaminated on hard braking.

 

In the late '70s and early '80s, many switched to the braided hoses but then began running into issues on the street. Unlike race cars where they are cleaned and checked frequently, on street cars the braided sections were collecting sand and dirt, which would wear on the hoses.

I don't know if that is an issue anymore, since that was 40 years ago and components and materials have changed.

 

Just not true. There is a huge difference!
it's certainly not about the look! No one see's em anyway!
it's about the inner liner that's NOT just rubber. That resists the puffing or swelling the rubber hoses do. It's about the fact they hold up longer, twice or 3 times as long. And don't collapse inside creating a one way valve that causes many a braking problem. They are NOT just rubber covered lines.