rosseich

I just installed a set of J56 calipers, pads, and brackets on my '72 LT1. The brakes are awesome, but the front calipers seem to be allowing air to enter the brake system. The brake paddle becomes soft within a week and then on hard braking the light will come on. I have installed several brake systems on C3s but have not encountered this issue.

Any help would be appreciated.

MelWff

How did you go about bleeding the system after installing the calipers?
Do the calipers have steel sleeves?
The calipers were complete assemblies with the correct pistons?

DUB

QUESTION:

Did you use a dial indicator and check the run-out on your rotors????

Also are you sure your bearing are set up correctly???

I would BET that if you check the rotor run-out..it is NOT going to be close to specs....thus....allowing the calipers to suck in air.

The reason I say this is because I have to fix brake problem like this often when that important step is overlooked.

Check this out. I make my own shims out of steel shim sheets to get them dialed into specs.


DUB

gerry72

Dub is most likely right on this one. I know on my '72, which I purchased long before the Internet was a thing, one of the first things I did was new calipers, master cylinder, and have the rotors lathed. I had the same issue; brakes worked fine but went soft after around 100 miles of driving. This problem persisted for a couple years with me bleeding the brakes once or twice a month. I finally solved the problem by replacing the rear rotors with new ones. While reading all the literature I could find, I later ran across a sentence that advised against lathing rear rotors on a C3 due to the particular specifications for rotor parallelism. You could find this out by following Dub's guidance.

DUB

I will say this.

Assuming that even if a new rotor being installed is going to be correctly indexed is an incorrect way of thinking.

Although IF a new rotor is installed and it indexes perfectly...GREAT. But I have run into many cases where this does not hold true..and it does not matter if it is on the front or rear.

I take out my dial indicator on EVERY job and check them when I am doing brakes that have problem in loosing pedal and NO visible fluid leaks are found anywhere. IF I replace rotors...they get checked. I ASSUME nothing!

Like I wrote...the wheel bearing also needs to be WELL within specs.

And it is regardless if the calipers are the 'O' ring design or the factory lipped seal design.

DUB

mrvette

I remember many many many years ago having used aluminum foil layers to make the rotor on the right rear run true....been that way since...I used 2-3 layers on one-two studs and less layers on the adjacent studs, and none on the opposing side, to keep any warpage down to zero when lugs tightened.....

rosseich

First, thanks for the information. I did replace all four rotors and believe the rear rotors are OK, but the front rotors are likely out of spec. I will need to purchase a dial indicator gauge and then check the run out, but I would like to understand why the problem occurs? How do the calipers capture air when the rotors are out of spec?

Thanks again!

DUB

I will write this.. I would NOT assume that the rear rotors are in spec. Check them all and get them in spec. Leaving the rear rotors unchecked is only doing half the job.

The calipers can suck in air..and they can also leak out fluid.

BUT.. when your rotors are out of specs. Every time the rotor turns one revolution...it can cause the pistons of your caliper to move in and out slightly.

I am by means a brake expert and know what is going on in regards to the physics at play that cause this problem you are asking about. I DO know hydraulic systems and how they must work. But I can visualize it...and I have SEEN IT. And when I wrote...'SEEN IT'. That means that I did not actually see air going into the caliper (obviously)...but rather the effects of rotor being out of specs.

YES...it can be a pain to do...but...you are working on your brakes.

But like I also wrote...if your wheel bearings are not well within specs...this 'indexing' of your rotor is a waste of time.

Like I tell my customers who run into this. I tell them the brake system of their Corvette brake caliper mounting design is not like 99% of the other cars out on the road. The caliper on your Corvette is solidly mounted to the bracket. So the caliper does not move at all. Many other cars calipers are mounted on guide pins that allow the caliper to slide on these pins. Other cars brake designs may have a floating rotor that can move.

SO...knowing that your caliper does not move...then getting the rotating mass of the brake rotor and bearing as 'true' as possible is PARAMOUNT to a good brake system that will be free of major issues.

DUB