66' Brake Problems

[Gary N.]

I have a 66' with standard disc brakes. I noticed that the brake pedal was starting to feel soft so I bled the brakes and it felt good for awhile then started to feel soft again after several hundred miles of use. I checked the master cylinder and I had not lost any fluid. I bled them again and the same thing happened. I did get a little air out of the left rear caliper though. I had replaced the rubber lines not to long ago and they still look in good shape. I next thought the master cylinder was bad, so I replace it with a new one and it did the same thing with the new master cylinder. I can not see any signs of brake fluid at any of the wheels. I am not using silicon brake fluid, but regulat dot 4 fluid. The pads are all in good shape. Could this be a caliper problem. I put new calipers on all four wheels with the stainless steel sleves some years back.

[John L.]

Re: 66' Brake Problems

You might want to check your front wheel bearings to make sure they are not loose.

[Jim T.]

Re: 66' Brake Problems

Gary with the information you have provided I believe that your loss of brake pedal comes from the rear. If your car has had the rivets removed from the the rear rotors and there is to much rotor runout, the runout is permitting the caliper pistons to move in and out. This movement puts air into the caliper which gives loss or rear braking. At a slow speed you might be able to hear the brake pads moving back and forth in the caliper.
I had the same problem on my 70 after Chevrolet mechanics worked on my car in the early 70's.
Fixed the problem by removing the spindles from the control arms and sending the spindles and rotors to www.bairs.com . Bairs rivetted the rotors to the spindles and trued the assembly just like the procedure that was used when my car was built. It has been about 20 years now, my 70's power brakes are still "outstanding".

[Joe R.]

Re: 66' Brake Problems

I agree with Jim that from the information provided, this might be a case of what is usually referred to as "air pumping" due to excessive runout in the rotors. There are two things that can be done to reduce air pumping:

1) Reduce the rotor runout to under .005 inches. One way to do this is to have the rotors riveted to the spindles and machined as an assembly, which is the way the factory did them. However, this is a big job if the car is all together and the only issue is rotor runout. Many people have had good success shimming the rotors with materials such as aluminum foil, thin sheet metal, or the nice-but-expensive ready made shims from Brake Align. If you check the archives on rotor runout, you will see much discussion on the topic.

2) It has been claimed that the newer O-ring style seals that Vette Brakes offers are less susceptible to air pumping than the factory style seals. These might help too.

My preference is to avoid riveting the rotors and simply shim them as needed. I like to have easy access to the parking brake without having to pull the spindle. The rivets perform no useful function once the lug nuts are installed.

[Gary N.]

Re: 66' Brake Problems

If I take off the rear wheels will I be able to see the run out by turning the rotor? I had the trailing arms rebuilt by Bairs some years ago. I don't remember if they re-riveted the rotors or not. I'll have to check. If there is some wear to the brake pads, would replacing them so that there is less movement of the caliper pistons be of any help. I would take it that the shims would go between the rotor and the spindle.

[Joe R.]

Re: 66' Brake Problems

Hi Gary:

You won't be able to "see" the runout, but you can use a dial guage to measure it. Anything under .005 inches is probably okay.

I should mention that when measuring runout on the rear rotors, two things need attention:

1) The mount for the dial gauge must be directly tied to the spindle assembly or the trailing arm, typically using a magnetic mount or some mechanical arrangement. If the dial gauge is based on the floor or the fender, movement of the trailing arm can affect the measured result.

2) Usually, you can not easily turn the rear rotors if the trailing arms are just hanging at full extension of the shocks, because the U-joints will bind. I had to support the weight of the car on the two lower spindle mounts, where the strut rods connect. A better scheme would be to disconnect the half shafts from the spindles, but this is a bit more work just to measure runout. If done carefully, you can measure runout without disconnecting the half shafts.


You are correct that the shims get added between the rotor and the spindle.

[Jim T.]

Re: 66' Brake Problems

Gary since you later post said Bair's rebuilt your trailing arms years ago, there should be no runout now. I would look for other sources. You mentioned that you replaced the master cylinder. Did you give the replacement master cylinder a good bench bleed before installing it? You have disconnected brake hoses, brake lines, etc. I would remove the master cylinder and bench bleed it if you did not do it. Then I would bleed each wheel, passing enough fluid that you have fresh fluid to each wheel and see how it holds up to driving.

[Gary N.]

Re: 66' Brake Problems

I have bled the brakes several times and get pretty good pedal for awhile. I don't think that there is any air in the master cylinder. Awhile back I had a problem with that left rear caliper sticking. I'm going to pull it off and take a closer look at it.