72 brakes

[Tom Hargrove (39845)]

I'm having a problem with the brakes on my 72. When I rebuilt it, I replaced the lines, pads, pistons, and master cyl. and booster. After bleeding the system, Ive had aproblem with the brakes coming and going. Sometimes there is a good full hard pedal, and sometimes when I start it up it takes a few pumps for the brakes to fully come up. The brake light comes on and the brakes are very weak for several stops, then It will improve to normal. I believe the front brakes are working, but not the rear, as a hard stop will lock up the front, but not the back. It seems to be getting worse, as now I have almost no pedal, and the light comes on when I push hard on the pedal, but still I have some brakes. I have bled the system twice. I'm thinking proportioning valve possibly. Is there any way to test it? Any other suggestions?
Tom

[John Lejsal (32031)]

Re: 72 brakes

Tom,

In the past I have encountered this same situation and every time it was always air in the system. It can be quite a feat to get all of the air out of the system especially the rear. If the brake light is comming on and you have front brakes the air is in the rear system. The proportioning valve is sensing less pressure in the rear and is moving to block the rear system so that you have front brakes. The valve is doing it job.

Keep bleeding and bleeding the system to get the air out of the rear. Push the pedal hard to help get the air out for it is trapped somewhere in the rear system.

Hope this helps

[Terry McManmon (3966)]

Re: 72 brakes

Tom,
John may be right in that it is just air trapped in the rear part of the system, however, air may also be introduced in another way. Usually air being introduced in the way I describe below will force the brakes on, and that doesn’t sound like what you are describing. It is none-the-less a good idea to understand how this can happen in order to rule it out as a contributor to your situation.
Corvette calipers (C2 & C3 versions) are a fixed design, that is they are bolted solidly to the caliper mounts. Therefore the run-out of the rotor must be minimal (I think it is .0005, but others will be able to verify that as I am working from memory and that is a bad thing for me to do). Chevrolet originally finished the rear rotors after they were installed on the spindle - that is why they are riveted in place. If the rear rotors were refinished off the car, or new rotors installed, or even if the rotors were removed and installed in other than the original position, the chances are that the rotors are out of specification. This will make the rear pistons a good air pump, filling the rear lines with air. If this sounds like your problem, the cure is:
1. Measure the rotor run-out to verify wobble, and either reinstall them in their original position or have the rotors refinished on the car.
2. Stainless Steel Brakes Company sells a wedge shaped shim to go behind the rear rotor to minimize run-out. You will have to measure first to determine which shim you need, and spend some time to install it properly.
3. Remove springs from behind the rear pistons.
If your problem is excessive rear run-out, you may have to do more than one of these to cure it. The same situation can happen on the front, but that doesn’t sound like it is your problem.
BTW: There is no proportioning valve in your system, if it is configured as the original brake system. The brass block that is mounted to the frame is a differential pressure device, and all it does is light the brake light if there is greater pressure in one system (front or rear) than in the other.

[Craig Schultz (29385)]

Re: 72 brakes

Terry - good overview of the air pumping issues. I think the rotor runout is 0.005" (5 thousanths) versus 0.0005 (5 tenths).........Craig

[Dick Whittington (8804)]

Re: 72 brakes

Additional note. Take a rubber mallet and tap the caliper that you are getting ready to bleed. It will help move the air towards the bleeders

[Jim Trekell (22375)]

Re: 72 brakes

Next time you bleed all four bleeders, two per caliper, remove both rear wheels and if you can safely, put the car in gear and watch the rotors turn. If any of the brake pads are being moved in and out with the roation of rotor you have enough run out to pump the air into the calipers. Or if you perfer you could just bleed both calipers and leave the house with good pedal and go for a drive down the interstate, the greater the distance will give a true test of what is happening. If you loose the hard pedal you left the house with, you have rotor runout. Usually new seals on the caliper pistons will give greater resistance to air pumping that seals that have had a lot of use.
About 14 -15 years ago I cured my problem on my 70, I only regret that I have about it all is that I did not fix it this way back in 73.
Are your rear discs/rotors still rivetted to the spindle/axle?

[Joe Lucia (12484)]

Re: 72 brakes

Tom-----

Are you bleeding from BOTH the bleeders on each rear caliper? There are bleeders on either caliper half for the rears.

[Tom Hargrove (39845)]

Re: 72 brakes

Thanks for all the suggestions. This is really a great board for those of us who don't have much experience with vettes (My first one). I will check the run out on the rotors tomorrow. They are not riveted to the hubs (weren't when I got the car) I'm not even sure that I got them back on the original sides. I will then go about bleeding the brakes until I get a constant pedal. Again, thanks to all of you guys for the help.

Tom

[Tom Hargrove (39845)]

Re: 72 brakes

Joe, your tip came in while I was posting. Yes, I am doing both bleeds on the back. I'm real curious to see what the run out will be. I'm thinking that this might be my problem, as I didn't check it while I was putting it back together ( I was having to turn pages as I went, must have missed one) Thanks again.

Tom

[Jim Trekell (22375)]

Re: 72 brakes

Tom since it is known that the rear discs are not rivetted to the rear spindle/axles and if you find runout, I can only recommend the best fix that worked for me and it solves the problem.
Send your rotors and spindle/axles to Bair's and have them rivet the assembly and true them, Bair's are the professionls at doing this.
If your front bushing on the control arm needs replacing and the bearings need servicing and you don't have the equipment to do it, you can send the whole control arm minus the caliper and Bair's will do it all. This will most likely be the last time you will ever have this done unless you put a 100K driving your car. Check them out using Google.

[Tom Hargrove (39845)]

Re: 72 brakes

Jim, I replaced the bushings and the bearings when I put the car back together. I have access to a good machine shop here so I will probably try to take care of the problem here. It will be good practice for the 69 big block I have sitting in my barn. It needs everything. Thanks for the help.
Tom

[Terry McManmon (3966)]

72 brakes and a few CAUTIONS

Following Christmas dinner I got the 1970 Service Manual from cold storage (the garage). It says:
"When mounted on bearing cups, lateral runout must not exceed .002 inch total indicator reading."
Consider that F70x15 tires rotate 775 revolutions per mile. At .002 runout the piston(s) are moving 1.55 inches per mile - 775 x .002 = 1.55
Given half this is one direction, and half another the actual pumping distance is a little more than .75 or 3/4 of an inch for each piston. Would I settle for that much pumping action in a mile? Nope. Do you want to?
All that said, I am impressed with all the comments this thread drew on Christmas day. Farther down it looks like Tom has a practical solution in hand: Check for piston movement while the disk is slowly turning.
TWO CAUTIONS:
1. Be sure both wheels are off the ground or you will find out whether the positraction works. Trust me, this is not the approved method for determining the functionality of the limited slip differential.
2. Some times the universal joints in the half shaft will go into a bind if the car is operated with the rear wheels at full jounce (hanging all the way down). Best practice is to support the rear of the car with suitable jack stands under the trailing arm.

[Craig Schultz (29385)]

Re: 72 brakes and a few CAUTIONS

Terry - thx for the correction. For some reason the 0.005 stuck in my head, and I thought you inadvertantly added a zero. Like you, I like to keep the runout to an absolute minimum, which is why usually leave my rotors alone when factory riveted together. My experience is these just don't wear much at all with factory pads, and the only thing the usually messes them up is someone "fixing" them by turning the rotors separate from the flanges. If this has been done, the only fix I like is reriveting and turning as a unit like GM did, getting the runout dead on.... Craig

[Terry McManmon (3966)]

Re: 72 brakes and a few CAUTIONS

I should learn not to shoot from the hip, but I haven't yet. I should have realized half a thou is too tight for lateral run out. In my defense, that is the limit for thickness variation, but with modern rotor turning machinery, in good condition, the thickness variation should not be even that much. Modern machines cut both sides of the rotor at the same time.

[Craig Schultz (29385)]

Re: 72 brakes and a few CAUTIONS

Terry - do you know if turning with the spindle attached as GM did and Bairs does can be done on a regular rotor lathe that cuts both sides simultaneously? I was planning to turn my own rotors on my Atlas quick change 12x37 lathe with carbide bits, one side at a time. I would think I would still end up with quite accurate end results...even though turned one side at a time, since mounded on centers with the flange and spindle, the process should work. I only doing this since bubba fixed my 67 rears before I bought it.....Craig